In many early computers, many of the chips were in sockets rather than soldered directly to boards, e.g. this series of pictures of the Tandy CoCo 1 has a note to the effect that all the chips are socketed: http://tech.markoverholser.com/?q=node/50

I can understand doing this with RAM chips, because it's often feasible and desirable to upgrade these; I think the CoCo 1 did often receive RAM upgrades.

But what was the purpose of doing it with all the others? I would expect soldering all the other chips directly to the board, would both save money and improve reliability by eliminating one thing that can go wrong.