Palpatine's aims in the films are pretty transparent and honestly do not need further clarification. He's a politician who wants power because power is a cool thing to have. He's also a member of essentially a religious cult that values selfishness and has a blood feud with the Jedi. Most of his behaviour can be attributed to one or both of these facts.
And whether or not Grevious is 'cooler', he's unquestionably more developed than Darth Maul because Darth Maul is a cipher who has all of five lines. Grevious has actual conversations with other characters, and a fair bit about him is established - a key Separatist general, a guy known as a Jedi-killer, and so on. Maul is simply the Apprentice.
What he shares with Maul however is that he's a filler villain. Once Tyrannus is ignominiously dispatched, the Separatists need a figurehead to represent them until that thread of the movie's plot has been resolved.