Primer is cheaper than paint. Some times you need more coats than expected for full coverage. I painted a pink room blue no primer no problem. I painted a brown room yellow with no primer and wish I used it because I needed way more yellow than expected.

Depends if you’re changing the type of paint oil over latex or drastically changing the color. Otherwise just paint it twice. Sometimes with primer you’ll end up painting 3 times. 1 primer 2 finish color