Why does my cookbook want me to drain my tomatoes for a pasta sauce? I have an excellent recipe for pasta sauce, but I'm having trouble convincing people to try it because it contains a counter-intuitive and potentially wasteful instruction.

The gist of it is this:


Take a tin of plum tomatoes
Pour the tomatoes into a seive and smush up the tomatoes themselves, draining away the tomato juice
Heat with a teaspoon of sugar and a generous pinch of sea salt


The resulting sauce is extremely addictive, but why do I have to drain the tomatoes? Tins of plum tomatoes aren't hugely expensive, but neither are they cheap, and I don't know why we'd want to waste tomato juice. Is it for the sake of taste (perhaps the juice is bitter?), or for texture instead?
The main purpose is to allow you to get a thick, hearty sauce more quickly.  By pre-draining the liquid, you lose a a very small amount of flavor, but save time and energy in reducing the sauce.  Otherwise, you would simply have to boil away the excess liquid to get the desired consistency.