I think it is often literally just for simple fun - certainly in the case of a man who does not find tickling erotic - I think he would tickle a woman for sheer fun, just messing about - but I there has to be some form of connection or chemistry there to start with, not necessarily sexual, but I don't think anyone would be drawn to tickling someone that they find unattractive or someone wildy different to them age-wise, or someone that they don't share other kind of laughs with. In the same way they wouldn't be playful in other ways with those people.
In my case, when I tickle a woman, the same as above applies and I will often tickle a platonic female friend in group situations as a fun/playful extension of our friendship and the social occasion we are sharing, particularly if her ticklishness is already know and provides amusement to everyone, but.....
Additionally it's a tool I use to establish a slightly more special connection with a woman that I've already started to bond/develop chemistry with, either on a one-to-one basis, or when we've perhaps separated slightly from the throng of a group situation. I will always have some sort of attraction to this girl, but not necessarily her looks and I wouldn't necessarily have any kind of romantic intentions towards her.
My reasons for tickling her are, that when the ticklish woman reacts, it's like a submission to the tickler - in that brief moment it's like she's saying "I'm yours now", it's an experience shared exclusively between the two people.....the way a ticklish woman may drop to her knees (then you help her up), the way she may fall in to your body, or reach out and tickle back or grab your arms in order to get you to stop - it creates a new dimension to the relationship and although it often goes hand-in-hand with romantic intentions, it can equally can just be a wonderful enhancement to a close platonic friendship.
Maybe it adds a bit of trust, or it adds an element of escapism to the friendship, keeps you both feeling young, maybe it's like telling a secret (ie. "I'm ticklish", ..."yeah, so am I!") - these things strengthen friendships, even if they do not lead to romance.
Cheers
TTG