For me, the absolute best use cases for serverless is for really infrequent, small tasks.

For example, I have a few data scrapers written in JavaScript but my regular stack is lamp.

So I don't have any need to run a node server 24x7 just for those once a day tasks.

But I have even found myself not needing serverless for that because everything is running in a kubernetes cluster. So I can just setup a cron to run them which launches the needed node containers.

So I guess in effect, I am just using a sort of self-managed "serverless".