We use Firebase at work (and I use it for my personal projects) for several reasons:

1. it is not just real-time DB with subscriptions but a lot more (storage for files, push notifications for mobile, cloud functions etc.) so it allows to build quite a robust solutions, but still possible to use just a minimal set of what you need

2. In most cases it's pretty cheap (unless you messed up you DB structure, reads/writes etc. - could be problem for a lot of traffic - so in such a case ready pricing and related guides properly), for side projects basically free.

3. offers free hosting with SSL certificates for static files

4. you can bootstrap functional prototype really quick and for the production, you do not need to worry about scaling.