You wouldn't think that a guitar string could really make a difference in your overall tone but they do something a little unique with these strings that makes a difference.
When I was a lot younger (and broker if that's a word) I used to leave the strings on my guitars as long as possible. What tends to happen over time is that the strings will collect dirt and oil from your fingers. As this happens the strings begin to lose their bright crisp sound. The debris from your hands will deaden the sound and your strings will make your guitar sound dull.
I used to have to change my strings before every gig so that I would have the best possible tone for my shows. I even went so far as to remove the strings from my guitar and then boil them. This works in a pinch to remove the oil dirt and debris so that they get some of their original quality back. The downside is that this is time consuming and only somewhat effective. So even though I was only pay a few bucks for a set of strings I found myself changing them every month or so.