Well there is the classic "Drums set the tempo" style of time keeping, the conductor is more a necessity in grandiose melodious works where tempo isn't easily kept from the percussion alone.

Rock Bands, some Marching Bands, and arguably a bunch of Jazz Bands considering how much those conductors tend to devolve into just dancing around on stage like a possessed Muppet, are able to keep time without the visual cue the conductor provides in a less rhythm intensive composition.

Not to mention that developing an accurate internal metronome for others to build a song off your cue is a skill unto itself, as well as being able to read every part of the song at once to be able to properly rehearse the combined ensemble and tell if someone needs an adjustment.