Enough that in 2019 it was the most popular topic at re:Invent (our big user conference) and that today per our re:Invent announcement almost half of all new compute workloads in Amazon are based on it. Pretty heavily used across different industries and verticals.

I rarely use lambda but I use alot of Google firebase-functions for majority of my server code. From my experience lambda/firebase-functions/Azure-functions are very popular.One simple usecase I can tell is payment-successful return-hook from payment servers like Stripe.Its a tiny task which just logs payment-success info and triggers a email..etc.

They can seem not that useful by themselves. Their power comes from the integration with the rest of AWS services.