You can use gels in a couple of ways. For one, you can gel your strobes to match the color of ambient light. That way, whenever you make a color temperature adjustment, you're changing all the light in the image the same way. It's much easier to do corrections when all the light matches in color. The second use of gels if for special effects purposes, like the image above. I used a blue gel on a strobe in the room behind the subject, and a CTO (orange) gel on the subject. The colors work well together and provide another layer of interest to the image.