On planets whats the point of population? why build habitation module and stuff?

They are basically the hit points for a planet. Same function as your ship's hull. More population, more damage it can take before being invaded.

awesome cheers mate

That's basically the planet's health; people need to get the population below 10 in order to invade.

Personally, I wouldn't bother putting population structures unless you have no weapon or shield research, because if you put attack or defense structures you'll lower their chance for a successful invasion. Population would slow them down, but invasion defense will keep them from taking it. (All attack/defense/invasion defense structures give you invasion defense).