In general it is a good idea to minimize the number of battery banks that are used. Typically, there should be a house bank, and a dedicated starter battery. Think carefully before adding more banks. However, small buffer batteries can be used to power electronics, particularly the radio, where both the radio and the buffer battery are mounted high in the vessel.
Some people insist on using a separate battery for the anchor windlass.
If used, this battery should be isolated so that it doesn't discharge in parallel with the house battery. A windlass only draws high current when the anchor is stuck to the bottom, so the need of a separate battery for it is questionable. On the other hand is a bow thruster that can draw high currents for extended periods. Peak load demands probably mandate a battery local to the thruster motor. A battery for engine controls is another case where a small battery can be used . . .