All dedicated gamers have their favorite series: Final Fantasy, Halo, Dragon Quest, Ys, Metal Gear, Splinter Cell. No matter which one they pick to be their personal favorite, there are always elements that tie each entry in the series together to create a cohesive entity.
In the days of 8-bit gaming, series were the result of a continuing story, or the additional adventures of a well-loved character. Games like Super Mario Bros. and Rockman (Mega Man in North America and PAL territories) flourished through brand recognition has the years went on. Gamers came to feel comfortable with a series, usually for gameplay, and sometimes even aesthetics or story.