solid_queue by default prefers you use a different db than app db, and will generate that out of the box (also by default with sqlite3, which, separate discussion) but makes it possible, and fairly smooth, to configure to use the same db.

Personally, I prefer the same db unless I were at a traffic scale where splitting them is necessary for load.

One advantage of same db is you can use db transaction control over enqueing jobs and app logic too, when they are dependent. But that's not the main advantage to me, I don't actually need that. I just prefer the simplicity, and as someone else said above, prefer not having to reconcile app db state with queue state if they are separate and only ONE goes down. Fewer moving parts are better in the apps I work on which are relatively small-scale, often "enterprise", etc.