They are going to publicly heap honors on Lepidus and make him a partner in their government because his good reputation will deflect some of the criticism that otherwise would have come their way. But Antony, using an extended simile, assures Octavius that Lepidus will merely serve like a donkey, which you load up with precious gold because you need him to carry it. But once you're steered it to where you want to go, you unload the valuable cargo and turn him out to pasture. That's just what Antony proposes to do with Lepidus once they no longer need him.