Bert, in very simple terms.
A normal developer like D76/ID-11 should give you a good overall balance of fine grain, film speed (box ISO) and good sharpness with an excellent tonal range.
A super fine grain developer like Perceptol or Microdol X does this at the expense of film speed, usually just Metol based they need increased exposure to achieve the same tonal range, but garon is significantly smaller.
A speed enhancing developer like ID-68/Microphen gives a higher effective EI at the expense of slightly coarser grain and slightly different tonality.
So choice of film, say Pan F, FP4/Delta 100, or HP5/Delta 400 and then choicer of developer Perceptol. ID-11 or Microphen gives a wide range of possible combinations and of course there are others.
It's why people say find a film/developer combination and stick to it learn all the variables.