There are a few reasons why women watch porn.
1) Curiousity
a) What's the fuss all about?
women watch porn because we want to see what the fuss is all about. Men love it so much and we want to to know why.
b) we're curious about a particular sex act/fetish. We never tried something and we wanna see if it's as good as we heard.

2) We wanna watch it with our man cos it might get us in the mood.

3) We happen to be feeling particularly naughty at the moment. But this is not the case everyday, it is a somewhat rare occurance.

But for most women porn is not masturbation fodder. Porn is not something that we can jill off to, however, we could watch a movie or check out a magazine and then use one of the scenarios to fantasize about while we are jilling off. The reason why porn does not make good masturbation fodder for women is that porn is made for men! Porn, as it is now, can never appeal to the average woman for that reason.

See also: porn is made for men