Welcome to my perfumed garden

I'm in the midst of writing an auction story and in the planning stage of a sequel to my last story, Chasing Rainbows and... I've never been a one for posting WIPs; mainly because, when in full writery flow, my stories are a constant, ongoing thing... I write, I hone, write, hone, go back, edit, switch bits about, hone again, write again - etc etc (you get the picture?)
So - I'm not really sure what the advantages are of posting a WIP? Yet, I see quite a lot of you do, so... I guess I'm looking for advice as to whether this is A Good Thing - or not?