Stranded vs solid wire
Solid wire (24 gauge) is good for use as jumpers in solderless breadboards.  Trying to push in stranded wire is frustrating.  Solid wire holds its shape, so you can route wires along a path and they'll stay there.

Use stranded wire for everything else.  It is flexible so it's good for cables going between a circuit board and knobs/switches on an enclosure, connecting cables between circuit boards, or any time you might have physical flexing in a wire.  24 gauge is a good all-around size for stranded too.