Well, I think fanservice by definition is unnecessary. The Japanese use the word "service" in katakana (and yes, they use this word exactly when referring to fanservice) when giving extra or free things, like a bonus. They put it in for it's own sake (well, for the fans that like it) and it was never meant to develop the plot or anything.

As for the reason they put it in, it is probably due to a number of factors. One is the practical reason of simple demand for it. Another may be the repression, or rather, the sudden lack of repression in Japan's youth population as of recent.

I personally don't mind fanservice, unless it is done very cheaply (Rosario x Vampire) or if it is the only content of the show (Ikkitousen). Otherwise, I consider it an extra treat that I could do without and ignore if I want to.