Helps simplify complex SQL queries and no need to waste network traffic on data that client side is never going to use, and waste CPU cycles processing it.
Yes, what about database portability?
I am on my 50s and it only mattered on a single project, which was anyway a middleware for application servers.
For sure, but have a solid grounding in set theory to go with it.
I've dealt with so many poorly-performing stored procedures that ended up being written as iteration over a CURSOR when they could have been done with sets. Programmers who don't grok set theory reach for iterative constructs which, while they work fine, are an impedance mismatch with SQL.
I have seen DBAs make wonders without changing queries, only by adding the right set of indexes.