It's a fair question. You're right, an 020 A1200 will run 99% of original games, in some cases better than an accelerated Amiga will due to compatibility. However as others have mentioned there are a few good reasons to upgrade:
1) Workbench speed - opening Windows with lots of folders (eg games, demos), redrawing the Workbench etc, much faster.
2) If you have a high-res monitor, you can run WB 3.5 or ClassicWB at a decent speed. The original WB 3.1 or lower look great at 320 x 240, but look terrible at anything higher due to the strange half-height icons, scroll bars etc.
3) I use my Amiga to run retro games from other systems like Scumm, Doom ports, Shapeshifter etc. Why you ask when a modern computer will emulate them better? The same reason I use a real Amiga instead of WinUAE - it's not just about the software, you need hardware that complements it. 90s games look good on low-res CRT 4:3 monitors, they look crap on super-high res wide screen modern LCD monitors.