Question about Android Market

When I went to clear the cache from my market, there was an option that I have never seen before. It says "Uninstall updates." Was there an update to the market itself, and if so what's so different? What will happen if I uninstall updates?

Thanks in advance.

[UPDATE]
The update, at least for T-Mobile users, allows you to buy with either your credit card (via Google Checkout), or it will charge to your T-Mobile bill.

 

I should also mention that if you T-Mobile account is past due it will disable the option to purchase apps with you T-Mobile account. It will force you to use Google Checkout. At least that's what the T-Mobile support people told me.