I like reading older papers. It allows me to develop a feel for the motivation of the current results and helps to introduce the culture of an area. But more that this, reading older papers sometimes provides insights which are taken for granted in modern textbooks and are assumed knowledge in modern papers. Finding that grain of truth that is occasionally overlooked via commonly accepted assumptions and simplifications is a real pleasure for me.
Of course sometimes I read older papers and discover that the material has been so successfully integrated in to the current literature that the main results and ideas are very explained much better than in the paper itself. I just wish there was a way to tell the difference.