Well, troop landings are a shield-skipping effect, so if you have planetary shields up, they can save you from an embarrasing loss (the enemy could capture your shield generators intact!)
They make your people happy! (Happy or else!
)
They provide hitpoints for your planet. Troop-sized shield generators provide 15 hp/kt, which is the best in the game.
So, a pile of heavy shield troops will not only provide an effective planetary armor, they will keep your citizens in line,
and will provide cover for your militia when ground combat does occur.