If you use the microwave a lot, you must have suffered a few times with food that is very hot on the outside but still cold inside. This happens because of the way it is heated.
The microwave oven, unlike an electric oven, does not release heat itself. What it does do is produce electromagnetic waves which stir up the water molecules in the food, making it hot from then on. The sauce will always heat up before the pasta.
In addition to keeping the appliance clean, the lid concentrates these electromagnetic waves closer to the food and heating is faster. This is where a very important feature comes into play: the holes in the surface of the lid.
The holes make the steam (water in its gaseous state) leave the energy concentration zone. Thus, it ceases to be the main target of heating, allowing the food to receive more waves and to heat more uniformly.
Make sure the lid you choose has a good amount of holes! This will even improve the performance of your microwave.
To better understand how the microwave oven works, check out the following video from the channel "Enementar, meu caro Watson", which explains physics and science in a fun and humorous way: