Your question is potentially mis-phrased.
Do you mean:
(A) "Why is 144fps at 144Hz smoother than 144ps at a higher refresh rate?"
or do you mean:
(B) "Why is 144fps at 144Hz smoother than higher frame rates at 144Hz?"
Do you mean (A), or do you mean (B)?
Either way, both are true, because framerate=Hz is always smoother.
-- The difference between the two creates a beat frequency (aka stutter).
-- There are 1 stutters per second for 145fps at 144Hz non-VRR.
-- There are 1 stutters per second for 143fps at 144Hz non-VRR.
-- There are 4 stutters per second for 148fps at 144Hz non-VRR.
-- There are 4 stutters per second for 140fps at 144Hz non-VRR.
-- The difference between fps and Hz creates a beat-frequency effect (aka stutters) on fixed-Hz displays.
That's why 240fps at 240Hz is better than 144fps at 144Hz
That's why there are stutters for 144fps at
>144Hz (non-VRR).
That's why there are stutters for
>144fps at 144Hz (non-VRR).
It's easy to see for yourself. You can witness beat-frequency stutters. Easiest to see if you use a microsecond-precise frame rate capper such as RTSS. Set an RTSS frame rate cap of 142 with VSYNC ON or VSYNC OFF on fixed-Hz 144Hz -- you will see a very regular cadence of 2 stutters per second in CS:GO or any software that easily runs capped-out -- such as older games with virtually zero frame rate slowdowns are excellent games to test beat-frequency stutters. 2 stutters per second is the beat frequency of 142 and 144. That's the difference between 142 (frame rate) and 144 (refresh rate). In-game framerate caps may show this too, but they may have slightly more erratic stutter due to a different
capping algorithm that might average things out more. If your frame rate varies a lot, you get erratic stutters.
Even stutters are visible at higher refresh rates. 239fps at 240Hz = 1 stutter per second. If your motion is fast (e.g. 4000 pixels/sec), a 1/240sec stutter is 1/240th of 4000 = a stutter-jump of 16 pixels. It's impressive how even a single framedrop stutter is still visible even at ultra-high Hz. In fact, at CES 2020, I was still able to see single-framedrop stutters for 360Hz VSYNC ON -- 1/360sec stutter is still (just about) human visible during VSYNC ON motion at very fast motion speeds. ASUS has a long-term roadmap to 1000Hz (thanks in part to Blur Busters), the final frontier is not 240Hz nor 360Hz.
Yep, what you call "stutters" are often beat frequency artifacts from the framerate-vs-refreshrate difference.
G-SYNC and FreeSync automates fps=Hz. The refresh rate changes in realtime to match game frame rate. You also want a VRR display with a wide VRR range to capture your entire frame rate range if possible. Thus, I generally recommend 240Hz+ if you want a low-lag VRR display. The great thing with variable refresh rate displays -- is there's no such thing as a single dropped frame on a VRR display -- it just doesn't exist -- it's stutterless framerate changes like
www.testufo.com/vrr -- 60fps goes 59fps goes 60fps virtually invisibly -- A tiny framerate change is completely invisible on variable refresh displays!
Even randomized framerates look perfectly smooth because of the above graph.
TestUFO:
Animated demo of struggling random framerates looking smooth
Look at how the framerates struggles to equal Hz, yet stutters don't show, thanks to variable refresh rate (G-SYNC or FreeSync). This TestUFO animation uses interpolation to emulate VRR in software. The monitor waits for the software to deliver a frame, before immediately refreshing on the spot (the monitor hardware slaves to the software whenever in VRR range).
TestUFO:
Animated demo of frame rate changes from 30fps smoothly to max Hz
Look at how it's done stutterlessly. 30fps, 31fps, 32fps, 33fps....[etc].....142fps, 143fps, 144fps. No beat-frequency stutters, because the Hz is matching fps, and the (erratic locations of) object positions are still in sync with your (analog) eye tracking positions along the axis of the motion vector. It's impressive how variable refresh rate just simply erases single-framedrop stutters, and tiny framerate changes. Sure, large stutters will show (e.g. disk loading stutters that lasts multiple fixed-Hz refresh cycles) and buggy game-engine stutters will still show -- but the typical single frame "missed refresh cycle" stutters are 100% completely gone with G-SYNC and FreeSync. Because there's no such thing as a missed refresh cycle when the computer monitor WAITS for your software to pass a new frame.
TL;DR: If your priority is smoothness, you want fps=Hz, either via VSYNC ON or via VRR.
Use VSYNC ON if smoothness is more important than lag! Or use variable refresh rate (VRR, such as G-SYNC or FreeSync) to automate fps=Hz while avoiding laggy VSYNC ON. On these displays, the display Hz automatically changes to match frame-rate in realtime, dynamically.
Now, if you use blur reduction such as DyAc or PureXP or ULMB, then your
beat-frequency stutters are amplified by blur reduction. A single framedrop stutter becomes MUCH more visible with blur reduction. There's no motion blur to mask single-framedrop stutters. They will show up that much more jarringly. You could go with an ASUS TUF series with variable refresh rate ELMB-SYNC (avoid the VA panels, stick to the good ones).
TL;DR: If your priority is smoothness with motion blur reduction, you may have to use VSYNC ON since most monitors won't do VRR combined with blur reduction.
Yes, it adds lag but makes blur reduction infinitely FAR better looking. To compensate, there are lots of low-lag VSYNC ON tweaks, such as RTSS Scanline Sync, NVIDIA Ultra Low Lag, and/or fractional frame rate
capping. And yes to smooth strobe-amplified mouse microstuttering/jittering -- with blur reduction you ideally have to jack up your mouse DPI to 1600dpi or higher (even 3200dpi), with an ultra-fine mouse sensor, very clean mouse feet, good esports mousepad, and ultra-low in-game sensitivity to compensate for high mouse DPI. It's best not to use old-fashioned 400dpi settings with LightBoost/ULMB, it stutter-crapfest for strobing during slower-speed and medium-speed eyetracked mouseturns. Use mouse profiles to use lower desktop DPI since your mouse arrow will be a rocket in Windows if you keep same DPI in desktop as your game.
See:
HOWTO: Using ULMB Beautifully or Competitively