Video games are an ever evolving medium. It may not seem that way right now, what with annual releases of the same ole same ole; I'm looking at you FIFA, Mario and COD. In years past, the rate of new ideas and new concepts coming into games was much higher, which makes sense since developers only really had their imaginations for inspiration. There wasn't this huge body of existing work for them to look at and be influenced by. So there was constantly new ideas and concepts being experimented with which sometimes could lead to new genres of games.