Monoliths do three things:
1. They give each military unit/mindworm one upgrade.
2. They repair any damage to any unit (not just military units).
3. If they are in the city limit, they give 2 food/2 production/2 energy regardless of the underlying terrain. Note: they cannot be upgraded except by a particular tech whose name I forget which comes late in the game.
On Edit: The tech referred to above is Manifold Harmony, which is only available in SMAX.