Scaling horizontally undoubtedly introduces complexity but it also comes with some upsides:
* DB backups are now (much) faster.
* Smaller backups means faster restores which reduces your RTO (Recovery Time Objective)
* If you have a well architectured application a catastrophic DB failure will now only impact a portion of your userbase instead of all of them.
There are probably more good reasons but these are the ones I could think of now.
Is high availability or easier backups why people look to horizontal scaling though? I don't think that's ever been a primary reason for any story I've read. It's a great "bonus", but I can't think that it would be a compelling reason to choose horizontal vs vertical scaling.
