Obviously lambda is a hammer, not a swiss army knife - you shouldn't shoehorn it into your app just because it's the new tech on the block. But the use case isn't niche at all.
We're using lambda for our service to ingest and process Hearthstone replays. An HTTP POST is sent to the API gateway, this triggers a lambda which parses a 0.5-2MB file, scrapes the data in it and stores data in the db + converts it to another format. The whole process takes 3-6 seconds. Lambda lets us easily scale to dozens of requests per second without having to worry about anything other than the DB server. Our frontend/app servers don't need to be touched.
The alternative would be to provision and manage a lot of EC2 instances to handle the load. We'd have to worry about provisioning them dynamically so that it's economically viable. We'd have to manage the load balancing ourselves. Lambda's been a huge time saver.
PS: Let it be known that Lambda only supports python 2.7, not python 3, and that really frickin sucks. Get your shit together, Amazon.