I’m sure I’ve missed it when I read the books, but why does Renly want to wed Margaery Tyrell to Robert?

Renly wants to wed Margaery Tyrell to Robert as part of a larger plan to oust the Lannisters – get Cersei out as queen, get Joffrey/Tommen/Myrcella out as heirs – and replace them with the Tyrells, with Renly likely becoming the next Hand of the King as the king’s loyal brother, the man responsible for uncovering Lannister treachery, and as a close ally of the Tyrells through Loras.