Our class What'sApp group is a lifesaver!
I'm not the most organised and we're good at reminding each other about things like Mufti day and invites to whole-class parties (which I have been known to miss in the past, and I expect my DC will still be reminding me about my failures on this account into their 30s!)
It's incredibly useful.
The purpose is it's a channel for parents to talk to each other directly, these groups aren't run by the school (usually) so it's a totally different thing to the school newsletter.
If your DC's classes parents are arseholes, that's a shame and of course it'd make a group chat a pain but you can always leave if it's not for you. But that isn't to be expected, and those telling you not to join for that reason are being pessimistic and prejudiced about groups of mums IMO (and it is mostly mums. We have 1 dad on ours, my other half!)
I've been on these groups for 4 different classes and they've been great.
Tracking down lost clothes is another regular post. ("little johnny came home with two identical-looking but differently sized shoes today, does anyone have the other one by any chance?"), organising chipping in for teachers presents etc
Or asking what this week's homework was.
Also it's good if parents need to speak to each other urgently about nice things (inviting everyone to the park on the last day of term) or difficult things, for example one of the dads in my DD's class sadly died of covid. The mum was able to tell us all via the Whatsapp group so we could have a chance to prepare our DC for hearing this news before they heard it in the playground and to know what was the best way to approach her DS about it.