Page 11 of the rules, section 5.3 ("THE REPLACEMENT PHASE"), case 5.31:
"During this phase, both players may now return previously eliminated ground units to play by purchasing them from the dead pile with Resource Points [...]"
Hence, you use RPs to buy replacement units. IIRC you can also lend resources to other powers. It's some time since I last played the game, so I can't remember if RPs had any other additional use (apart from events / replacements / lend).
Found it. Thanks very much. Maybe I need stronger eye glasses. hahaha