You could ask the same question replacing "switch" with "router" or "access point". The reasons to replace the vendor firmware are pretty much the same.
OpenWrt gives you timely updates, more features, better configuration system, infinite flexibility, and freedom from any cloud lock-in etc. Whether any of that is required will of course be up to you as the user. There is no doubt that most users are completely satisfied with the vendor firmware on both switches and routers, even if I believe OpenWrt is so much better. That's a personal view.
Coming back to the switches. In my home network I have two realtek based switches - a Netgear GS108Tv3 and ad ZyXEL GS1900-10HP. I also have 3 other switches - a Cisco WS-3560CX-12PD-S, a Cisco/Linksys SLM2008 and a Cisco SG250-08. All 5 are small (8-16 ports) managed switched working perfectly fine with the vendor firmware. Ehey are all as cheap as they get without dropping features, except for the 3560 which is over-priced, . The vendor firmwares have all the features I really need, and most of what I consider nice to have.
So why am I running OpenWrt on the two realtek-based switches?
Well, the Netgear is easy: You can't access the full management UI without registering an account with Netgear. This is an artificial lock-in. There are no cloud features involved It's all local UI. TBH, I haven't explored much of the vendor firmware of this one. It was bought to run OpenWrt, and works nicely with that.
I did use the ZyXEL GS1900-10HP. with the vendor firmware for a while, and it was "good enough". But there were a couple of annoying issues. It crashed/rebooted occasionally. Not very often, but often enough to be noticeable. Maybe twice a moth or so. Been running OpenWrt on it for 6 months now and haven't seen a single unexpected reboot. Another problem was that the fiber link always needed a helping hand on reboot. The switch failed to bring it up unless I did an down/up toggle on the other end. This bug might be related to the dual-rate SFP+ I have in the other end, possible causing a rate confusion. But OpenWrt has never had any problems with the same hardware. All ports, including the SFP ports, are brought up just on reboot. The hardware is exactly the same in both ends. So the vendor firmware issue was definitely just a software bug.
Yes, minor issues only. But still: OpenWrt is significantly better - for me and my use case at least.
Personally I also find it much easier to manage an OpenWrt device using ssh, than having everything hidden behind a device specific web UI or a Cisco IOS-like configuration language. But I guess that depends on what you are most familiar with.
Then there is long term maintainability. A switch is not something you replace every second year. I bought the SLM2008 in 2009 and I see no reason to replace it yet. It's probably been 10 years since the last firmware release. That exact model has been replaced with a similar one based on newer chips several times since then. The SG250-08 is actually a newer version of the same, based on entirely different hardware. Sure, these are switches and I don't want the world to hammer on their management interface in any case. But I would have felt better if it was possible to keep them updated, like OpenWrt allows.
The vendor firmwares are definitely feature packed. But that's nothing compared to the flexibility offered by all available OpenWrt packages. There obviously isn't a CPU capable of running heavy stuff, but there are lots of small things you can make your switch do. Only limited by imagination.
I could continue forever. But I've probably long lost everyone, so I'll stop here for now