I bought a small one in the process of acquiring standard tooling for a
new minimill, but am a bit vague about the correct use of it. Conventional
fluted mill seem to fit most of what I'd be doing, and flycutter is for
finishing a surface. So what is the indexible carbide cutter best used
for ?

You really need a powrful rigid machine to successfully use carbide insert
tooling. Me thinks your machine can't use it to advantage. For insert
tooling, you just replace an insert when dull and you're back to a brand new
cutter with the exact same dimensions. No regrinding or changing to a new
cutter and having to re-establish all your offsets. Big time saver.

For small one of jobs like most hobbist have, regular endmills end up
cheaper. You almost cry when you schmuck a $150 holder with $40 worth of
inserts into your brand new vice and almost finished part. If you haven't
schmucked an endmill yet, you're not a machinist.

Karl