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:

1. Take a tin of plum tomatoes
2. Pour the tomatoes into a seive and smush up the tomatoes themselves, draining away the tomato juice
3. 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?