Those gels are used to match the output of the fill flash to the interior lighting in particular situations. You then set a custom WB based upon the interior lighting and you're ready to shoot.
Otherwise, if you mix interior lighting with fill flash, you'll get a real mess, since the flash is "daylight balanced".
There are some interesting alternative techniques, too. For example, using a fill flash outside, use a gel so that the flash puts out a tungsten balance, and set your camera's WB to match... that will make all the background area go sort of bluish, and make the subject lit by the flash primarily really stand out against it. It's a neat effect, but can be overdone if not careful.
Experiment!