Its a habit of old, back to the days before there was such things as quote tags and php forums. Early forum software used it in place of quotes.
In fact some forum software until quite recently still used it and you could change the colour of the text depending on how many > you used to denote multiple quotes.
for example > would change the text to blue, >> text to purple >>> text to green
I think UB and a couple of the older regs will recall at least one example of such a forum.
I guess because I still do it, it gives away the fact that Ive been a nethead for @20yrs.
It's actually a fallback to the same system that email used too: - in that ">" denotes the senders message.
I tend to also use it a lot when on the ipad, because it takes just too long to highlight bits to c&p and try line up tags.