There are two reasons to do this: the first one is that you can really save your pasta if you have dried it too much, for example if you are tossing it in a hot pan with other ingredients or simply if your time management is not so perfect, as sometimes happens when cooking.
The second reason has to do with chemistry: the water in which you boil the pasta collects starch. Once heated again it expands and mixes very well with the gravy and the pasta. Get a sumptuous presentation and a pleasant creamy feel to the palate, not an oily one, but much lighter!