If you want higher resolutions and frame rates, you need more powerful GPUs to handle it. For example moving to 2560x1600 or to 120fps doubles the pixel requirement over 1920x1080@60fps. So whatever amount of power you needed to achieve 1080p60, double that for either of those targets. 4k will require a quadrupling, and 120fps 4k would require 8x the power.
All this is assuming you are getting 60fps in the first place. Now maybe you are fine with trading off lower frame rates, or lower resolutions, that's all up to you. If 720p30 is your target, you can get away with a whole lot less power. However that doesn't mean that nobody wants to target higher resolutions or frame rates.
There are also other visual quality settings to consider, like anti-aliasing and so on that can require more power. Depending on what you are targeting with that, you can need a lot of power.
Personally I really find frame rates much below 60 pretty annoying in most games. I really like the feeling of fluidity you get. 120 fps is even better, but the monitor I normally use doesn't handle that. Well maintaining that 60fps at a 2.5k resolution is not a trivial feat. I don't think a $250 graphics card would do that for most games.