If you have not take part in any contest. You have no idea if you have the chance to win. If you own a blog and email account. You can get started to take part in contest. Who knows you might be a lucky winner.
Some contest required to have a PayPal account because there are Prizes which involved Cash. The best way of getting the money safe and sound is getting them via PayPal.
Taking part in contest, if you won ad space this will be the chance for you to use it to promote your blog. If you win a review on your blog, you will get to know if your blog need any improvement or not.