How do you manage multiple instances of the application and not introduce split brain?

> Or you could do it at disk level (RAID).

That is not comparable. RAID's redundancy is not the same as the redundancy in a multi node database cluster. You have one service, not multiple services, network card gets fried, your database goes down, you can't promote a standby to master and be on your way. Also RAID is a single disk as far as the OS is concerned, so you could hit I/O limits (especially if you have a single binary) that cause your app to chug, you cannot split your writes and reads across different physical or virtual machines that have different disks.