Whether you want to record in the studio, play a gig or just rehearse, you will find that you are going to need a certain amount of equipment. Just how much will depend upon the circumstances you are in. Instrument players will need their amplifiers, vocalists will use a microphone and for rehearsals and gigs a PA system.
The size and output power of the equipment will depend on the size of gig you are doing. In a studio a PA system will not be necessary as all the sound will be processed by the sound engineer. Using the correct rehearsal equipment will ensure that you get the best results and achieve the sounds that you are looking for.