I usually like to do something that builds up to the next part of the song. From intro to the drop, definitely. Usually leading up to every hook. Sometimes its drums, sometimes sweeps or reverse cymbals. It depends what kind of feel you're looking for in the song.
Sweeps and risers can either be the start of a new energy, or the end of the current energy (you can cut it off to nothing, or drop down to a mellow part).
Drum fills add polish to the song, IMO.
You just want things to flow. If your changes are dry and abrupt, it may not be enjoyable to the listener. I've seen it referred to as "ear candy" - little sweeps and cool effects that keep the parts fresh and progress the song.
If you mean "transitions" as in changes, like verse/chorus/bridge and that kind of thing, well that's a case-by-case thing too. Some progressive EDM and rap beats can use one part throughout the song, and just add layers to make verses and hooks stand apart. But the old Pop standards dictate things like Verse-Chorus-V-C-Bridge-C and that type of stuff. Adding pre-chorus or instrumental parts is solely up to you and what your song needs.