The subject pretty much says it all. I just can't find any effect when I have population increases.... what exactly does more population on a planet do?

Population is the equivalent of Hull for a planet. The more population you have, the longer it will survive when being attacked.

aha ty for the info.

A point of note, is that anyone attacking your planet can kill 20 pop max, no matter how high their attack.

Another important point is that there is an artifact that kills half the population on a planet, so most people only put normal population structures on a planet that they know is under attack and that they have used a Planet Vortex Deflector on (which makes it immune to artifacts).