Any Autobot needing competent repairs and unwilling to be used as a lab rat can call upon Fixit. On the bright side, he won't subject you to dubious upgrades the way his colleague Ratchet will, and does better work than his other colleague Hoist. On the down side, he doesn't work for charity. If you want to get fixed, it's gonna cost ya.
Rodimus told Ratchet that he'd rather do his own repairs or pay Fixit's price than get treatment from the crazy medic. Unfortunately, he'd already been "fixed" by that point.