Shooting photos on a particular theme can help provide inspiration and get you out of a photographic rut, giving you inspiration for your photography. Sticking to a single theme will also force you to shoot more creatively, looking for photographs that meet your theme where you might not have seen a potential photograph before.
Examples of themes are:
A specific color
Materials & objects
Image text project by Joost J. Bakker IJmuiden on flickr (licensed CC-BY)
A certain location
Specific type of building
Part of the body
Really you can choose anything as a theme. You don't need to choose a specific type of subject to photograph, you could choose a more conceptual theme, such as love or anger. You might also want to try a theme based on a certain type of photography, or maybe taking all your photos with a certain lens / focal length you don't use much.
There are also plenty of theme and challenge groups on the web that you can join. Generally these groups will set a theme, then you have a set amount of time to take the best photo you can that satisfies that theme. Often you can upload your best photo to the group and members of the group may comment or vote on which photo they think is best in terms of the theme set.
This can be helpful in a number of ways:
The challenge / competition aspect encourages you to work harder to get the best photo you can than if you were just shooting for yourself.
The comments and votes from other members of the group can often give you tips on what is wrong or could be better with your photo, giving you valuable advice to improve your photography.
Looking through the other photos of the group that you like best, you can analyze the photos and see what makes them work, then use this in your own photography.
Examples of Photo challenge groups are:
And there are literally thousands of other photography challenge groups.
Another advantage of shooting photographs along a specific theme is that presenting the themed photos together as a group can give the photos more strength than if they were presented as individual photos. This is why photography exhibits are nearly always themed around a certain subject, rather than just a collection of random good photos.
If you're looking for a challenge to improve your photography, shooting to a theme is a great way to do it.