Doing this has several benefits I've found:

1) Makes you appear aloof and not needy

We all know that showering a beautiful woman with attention will not win her heart because she is used to men leering at her and catering to her needs. Women also generally have a good idea of how attractive they are compared to other women. Therefore, it will throw a smoking, fine vixen off when she notices you conversing with her less attractive friend and not her. Unlike her attractive friend, the unattractive girl is probably not used to as much attention from men and might receive your conversation more openly.

2) Girls you are not attracted to can make good wing(wo)men

We've all heard it before: a girl can make the best wingman. If you get on the good side of the unattractive girl in the group, she will put in good word about you to her hot friends. One of my best friends to this day is a girl I am not attracted to (though she has no problem getting other guys) and she has hooked me up with multiple friends of hers that she knew I was attracted to (or knew they were attracted to me). These kind of women will especially do this for you if you can find dudes for them, I've found.

3) Being nice to unattractive women demonstrates value

A woman wants a man has something to offer more than looks and witty one liners (though to some degree these are also important too). So if she sees you are interested in bonding with/getting to know her unattractive friends, she will see you as unshallow, caring about more than just getting laid (which also helps with not appearing desperate) and just an outgoing person in general.

4) You could make a great friendship out of it

I realize this part of it has less to do with seduction, but there is nothing wrong with making a cool female friend that you have no intention of shagging. There is a lot more to someone than their looks. Part of seduction is having a healthy social life to begin with, so adding someone unique, helpful or that you simply connect with to your social circle certainly won't hurt your luck with the ladies ;)