I always thought the monoliths in 2001 were one of two things (maybe both) --
(1.) Alien devices designed to accelerate evolution on Earth. That is, the apes/early hominids that interacted with the monolith got smarter, learning how to use bones as a club for a weapon, and therefore they won out against the competition. Yes, the part at the beginning with the people in ape suits was important.
(2.) Communications devices left as "tripwires" by alien species to keep tabs on the progress of Earth's evolution. That is, when the monolith detected its disturbance on the Moon, that symbolized humans had made it that far in space. When they made it to the one near Jupiter, then that symbolized another milestone.
Once they hit a certain level, then other interventions and processes could begin.