The holes stop the lights from filling up with water and circulate air. If you have a big condensation problem then you need to dry out the lights completely. This is not an easy task. I used a rag on the end of a wire coat hanger and a hairdryer. If the moisture is between the clear and the coloured lens then there is not a lot you can do without stripping the lights out completely.
If your drilled holes are in this area then try warming up the lens to try to dry it out. You need to do this in a warm dry area otherwise when it cools down it will only draw in more damp air.