One of the main reasons is so the band can hear themselves. If it is only going from the audiences’ speakers, the band can hear the reflections off the back wall so there are timing issues. Also, the band often require different mixes of sounds to the audience. In larger setups, the bassist’s monitors may be separately mixed from the guitarist’s – the bassist might want the kick much louder in their mix, while a singer might want the melodic instruments more prominent so they can stay in tune. In smaller setups, it is just the one mix for the whole band, but they need close monitors so they are all in time and can hear each other play cleanly. Monitors also mean that they have a good sound instead of the dulled sound of outward-facing speakers