It's a good question;
Primarily depends on the rest of the setup. If you were VR, multi monitor setup or favorite game is MSFS 2020 or 4k high hz, then the rest of the setup will be easier to see/feel the difference if sat comparing 3090 - 4090.
You cant see the benefits on youtube of 4k or high hz if you only have 1080p 60hz monitor yourself, hence why results are numbers based rather than actual benefit/feeling based as people and the stream may not actually be seeing - capturing accurately every frame. Youtube probabaly cant do 4k 120fps anyway to portray the effect.
A gfx card purchase should be made regarding your monitor if a general gamer and/or a specific game if you play it a lot and depending on how serious you are about it. Casual gamers not so much or may lead towards a console.
Cockpit games under VR or multimonitor require beefy GPU's especially for games that are hard to run anyway.
Higher resolution always looks nice
VRR is also very nice to have
High hz screen is also a nice game changer for smoothness or comfort, then theres the CPU and sometimes RAM in being able to feed the fps to compliment the refresh rate of your monitor. I havent seen +144hz myself (e.g) 240hz and would have to see it in person to see the difference. You just cant see it on youtube or even if 240 fps / hz is possible via online videos and would probabaly need to be downloaded as a lossless file to see - then you'll need the equipment also.
CPU & RAM wont bottleneck your GPU - CPU's, especially 1% lows can make a really noticeable difference to game smoothness - especially when paired with VRR where microstutters are less as the VRR range the GPU is working across is less.
So unless you have a high hz, high resolution screen, with hardware that wont bottleneck your GPU, you wont get the full benefit of measuring a 4090.
90 fps is much better than 60 fps - paired with VRR and a 120hz panel. 4090 can run most games at 4k ~120fps.
Then there are technologies such as raytracing on top.
Only way you can really see say 4k, high hz, high fps, 1% lows, high res VR setup, is with your own eyes. Otherwise you have to rely on numerical results and bar charts. Imagine going to CES (any EXPO) or OCUK shop you could see in person.
Games themselves depending on how well optimized they are or technologies implemented such as VR can hammer a GPU.
Take CP2077, FC6 or MSFS 2020 - a 4090 will be easy to see the difference over a 3090 especially if RT is an option - then there are various levels of RT.
What is your personal setup? Monitor in particular.