Why so many kinds of developers? Much like film of varying speed and grain characteristics, developers are no different. They vary primarily by the developing agent or type and how it operates on the silver crystals of your film. Some are of a solvent type, which tend to reduce grain by eating away at or rounding the corners of the grain. The downside is that some sharpness is lost. Other developers are of a non-solvent type which holds sharpness, but as a result tend to be grainier.

Choosing a B&W Film Developer | La Vida Leica!

A good exploration of the different properties of developers and why you might choose one.