Why do we need to type characters from a picture....

We will b asked to type characters from a picture in most of the online transactions starting from creating accounts to change passwords etc.. I always having a doubt y do we need to type like that... what security they r achieving with that...
here z the reason...
You must type the numbers and letters you see in the picture to confirm that a person is trying to access the page, rather than an automated program. This helps keep automated programs from creating fraudulent accounts or misusing accounts, for example, to send large amounts of unsolicited e-mail.
In most cases, automated sign-in programs can't recognize the numbers and letters in the picture.