It really depends on what You are raiding. Raiding a "pop-up"-base of the pirate or luddic path faction makes little sense, but raiding an actual colony with the "heavy industry" building can yield blueprints used by that faction and even special items (nanoforge, synchrotron) for Your colonies. The raid dialogue usually gives You a hint like "some blueprints may also be found" or something. Also, You can always inspect a colony before raiding by finding the corresponding dialogue option (don't remember where exactly) so You can see how rich the colony is, and in what areas of production it may have stock. It's rarely worth the effort for just ressources, though, in my experience.
To make raids easier look for ships with the "ground support" package (I think the valkyrie mid-line crew transport has it). Also, You can raid again and again and again, however, You have to disengage and wait around for a day. (The dialogue actually says that. Not to be antagonistic here, but it really helps to read those lines, Alex rarely throws useless fillers into his strategic messages.) The advantage here is that the stability hit from the raid before makes the next raid easier. Downside, there is less and less to gain each time to cover the cost, so it only really pays if You are looking for a specific blueprint or colony item.
Finally, raiding destabilizes a market and that means You can gain market share, have an easier time with that faction's fleet (if it's a ship spewing world), better warez at the black market etc. - if You terrorize a place long enough You might even manage to "decivilize it" which turns it ready for colonization. As far as I know there is no "invasion" option though. You can only take over a colony via long-term terrorism and then re-erect it with a +25% hazard modifier.
