I found out at 16 weeks with both of mine. I just think it's nice to be able to concentrate on one set of names, picking out prams, cot bedding, little sleepsuits etc. Sometimes there are massive bargains that you can't resist but miss out on if you don't know the gender. I don't go massively overboard with gender specific but it's good to have an idea of what to look for.
I have a lot of plain white sleepsuits and white and grey stuff, just in case baby isn't a boy like they said lol, also have a bit of stuff from when my daughter was a baby so in that respect I will be covered, but it is just nice knowing for me, I am far too impatient to wait, it's hard enough waiting for an actual baby lol.
As for bonding, I guess it makes things feel more 'real' for me, referring to baby as he or she, gaining a bit of control in my life when my body is being taken over lol. I also refer to baby as the name I've chosen and if baby is still called that when it's due then I know I have picked a good name lol.