Why plane your intakes w/ milled heads???

They don't know that the heads have been cut on the chamber side, cuz they picked them up at a flea market, the shop that cut them didn't mark them for how much the cut them, nor did they do a compensation cut on the intake face. Some shops won't do it unless you ask. There's alot of reasons why people cut manifolds.

In the end, it's to make the manifold fit.