Planetary defense measures.

[Baron Munchausen]

WPs are certainly best because they are ON the planet. But satellites are still useful, especially if you use missile satellites with PDC on them. The missiles have good enough range to be useful even if they're on the 'wrong' side of the planet, hit just as hard if they come from a sat or a WP. But putting them in the sats saves planet space for the direct-fire weapons using highly advantageous WP mounts. PDC above the planet also adds a lot to defense against long-range missile attack. Your WPs still ought to have some PDC of their own, of course. And the WP weapons are good defense against attempts to take out the missile sats. A deadly combination!