Do you need a space port on a planet to move resources to and from it?

[Krafty]

Eh?

[ASHBERY76]

No, but you want at least one space port on each system.

[Duckfang]

Not on a colonised world.

Space ports sort of act as central storehouses for resources within your empire; freighters will pick up mined resources from your mines, gas stations and colonies and then deposit them in space ports where they can be sold to other empires or taken to shipyards to help build ships, etc.

Space ports also tend to have some shipbuilding facilities and - I think - allow more ships at once to dock with them than just landing on the planet does (depends on the number of docking bays a port has, afaik). They can also provide research facilities as well as recreational and medical facilities to the colony (which makes your people happier).

EDIT: It's worth noting that for a fledgeling empire they're also very expensive to maintain and can crush your economy if you overextend yourself and build too many of them. I usually build a small or medium one in a cluster of stars I'm about to start colonising so the supplies can be shipped more easily to the developing worlds. I like to build them near alien races too so I can invade their economy and make them dependant on my trade. [:D]