With a history going back through six centuries, the viola has been composed for by history's greatest musical minds and earned a permanent place in the symphony orchestra. It's a crucial instrument in countless compositions, and it's fair to say that strings are as important to the viola's sound as the viola itself is to the sound of the symphony. There's no other single upgrade you can make that will affect the tone as much as changing your viola strings, and since they're in contact with the bow the whole time you're playing, they're also going to show some wear over time. Considering those two facts, there are lots of reasons to change your strings - from customization to simple maintenance.
Historically, viola strings were made from animal gut, but modern materials have given us some more reliable (and humane) alternatives. They've also added more variety, which you can see in strings like the D'Addario Prelude Series Viola String Set. Based on a solid steel core, these strings are designed with a nod toward students and amateur players. They don't stretch or contract with temperature and humidity the way gut strings do, and they last much longer, so you'll get a lot more use out of them between changes.
If you're playing at an advanced level, you might prefer to pick your strings one-by-one instead of buying a full set. This gives you a ton of control over your sound across the scale, and there are plenty of single-string options here to choose from. Take the Pirastro Aricore Series Viola C String for instance: its synthetic material has a darker timbre and warmer sound than a steel string. Another great example would be the Pirastro Evah Pirazzi Series Viola A String, which produces a complex tone with responsiveness that can really bring out the heart and soul of your instrument and playing style.
No matter how long you've been playing viola or what sort of violist you happen to be, the right set of strings - or individual strings - will be waiting for you somewhere in this section. All you have to do is find them! Start by deciding what core material is the best fit for your needs and go from there; as long as you take your time and make an informed choice, you can look forward to results that might even make you fall in love with the viola all over again.