Understanding who built the monoliths and what they are does not fully explain their purpose or how they work. For example, the lunar monolith sends a signal to the monolith near Jupiter, but why? To get a full picture of these machines, it is necessary to go back to the very first one in the movie.
After encountering the first monolith, a group of ape-like early humans begin to develop deeper intelligence, using tools and weapons to become the superior group and defeat their competitors. In this sense, this monolith serves as an accelerator that encourages the evolution and progress of those who find it. It essentially gives humans a nudge in the right direction to ensure they are on the right evolutionary path (via Film Site).
The lunar device has a similar purpose, although more subtle, pointing humanity to Jupiter so they can continue their journey to their final form. It also acts as a beacon and test, as it can only be discovered when technology on Earth has advanced sufficiently, telling its creators that humans have reached a point where they can travel in space.
On a deeper level, some interpretations of the film suggest that they could also represent our own dependence on technology. The monoliths, with their rectangular shape and black coloring, could be a symbol for screens, according to film analyst Rob Ager.