Honestly, the biggest reason I could see for upgrading to this one is a better webcam since most people using a Mac have likely been working from home and bearing Zoom calls aplenty. But I'm also a bit apprehensive about this model, since I'm not sure the upgrade is justified when the benefits are so small.
If you use your iMac for work and your work involves significant use of Zoom, then I would argue that looking and sounding much better on those calls is a very significant benefit on its own. If the M1-driven MB Air and Pro are any indication, this will also be the fastest iMac by a wide margin--probably the largest speed upgrade in iMac history. (And think of the speed when most apps are updated to run native on the M-chip.) Plus it adds Touch ID, WiFi 6, a better audio system, the latest bluetooth and a 24" screen in the same overall dimensions as the former 21.5 inch.) But yeah, what a DISAPPOINTMENT!
Unless you're working near the bleeding edge of video editing with a need for more speed and a bigger screen... or you just have to have the fastest toys out there... the 2021 iMac will handle what nearly anyone else needs it to do. I'm replacing my mid-2017 BTO iMac 27" that I spec'd with the fastest processor, upgraded graphics and SSD -- for less than 2/3 the price of what that cost me, I'll have a significantly faster and better iMac all-around. (Plus, Apple's trade-in deals are pretty generous.)
I own an iMac 2011 27". (20GB ram, 256 SSD, not an HD drive at all, hugh Sierra) using it for FCX Pro (mostly 1080, rarely 4K), audio editing, photoshop and (via skype and messenger since 2016) teaching chinese.
I intend to by the new iMac 2021 24" because I can not wait any more for the new iMac 2021 30". I don't think I need 30". The 27" was sufficient for my needs.
Let's see what the 24" can do . . .