The advantages of joining a player run corp are numerous, but there are disadvantages. The advantages are:

1) The corp hangar.
The corp has a joint hangar, accessed via any station that has an office of your corp, where you can share things like ammo and mods that other people no longer want, or has been deliberately manufactured the the corp members.

2) Looting and Salvaging.
You have the right to open and tractor the wrecks and cans of anyone in your corp, whether they are online or not.

3) Standings.
NPC corps are often set to red (i.e. shoot on sight) status by player run corps because, as they can't be have war declared upon them for their actions, NPC corps become a haven for greifers (pirates who attack others just to annoy other players rather than economic or political gain). Also NPC corps never achieve the standings needed to set up a POS, or set up jump clones (although larger player corps have trouble with this too).

4) Teamwork.
Probably the most important one, player run corps tend to have a strong sense of teamwork and are more willing to invest time in new players. People in NPC corps have very little to gain from this, and tend to be very solo players with little interaction between players.