Hi Razvan,
I'm only aware of "SQL Server" reasons e.g.: Performance, backup/restore time, separate drives etc. If you have a few high volume processes you could create different databases for these.
In the past it was possible to export a process from one database and import this and not a template it in another. Today this is no longer possible, unfortunately. This could be used as an alternative to business entities. For example when it was required to separate the data in a physical way. It's less likely to accidentally show ( sensitive) data from another business entity if it doesn't exist in the same database. :)
Maybe there are even legal reasons when you need to separate the data if you have companies in different countries.
It will be interesting if there are any further reasons.
Best regards,
Daniel