There are arguably four roles that studio monitors play, and larger studios will often have two or three types of specialized monitors.
The most well-known are mixing monitors. These are typically small and used near-field, and as the name implies, they are used by the engineer to mix the individual songs. Midfield monitors may also be used as mixing monitors, as they are larger and have better bass response than small nearfield monitors.
Main or mastering monitors are usually much larger and are usually positioned farther away from the main mixing position, close to or even in the wall. Sometimes midfield monitors are used as mastering monitors.
Tracking monitors are less well known, and are what the engineer listens to in the control room as the musicians are playing in the live room. It's valuable to hear accurately what you are recording in order to know you're capturing what you want. Often the mixing or mastering monitors are used for tracking, but sometimes dedicated tracking monitors are used. I have been invited to build passive tracking monitors on occasion, with one advantage of passives in this application being zero latency, and therefore perfect synchronization between what the engineer sees through the window and what he hears through the speakers. (I wouldn't have thought a few milliseconds of latency would matter, but have had customers tell me otherwise.)
The fourth role of studio monitors is, "impress the client", said client being the musicians who are paying the studio. This is usually the job of the main or mastering monitors, when the band wants to hear what their newly-finished album sounds like.
Here we see small nearfield monitors on the console; midfield monitors on stands; and main monitors flush-mounted in the walls. I think the mains are used for tracking but I'm not sure:
View attachment 351634