What is the logic behind mixing longer runs with shorter runs?

On the programme I am currently following each weekend I have a long run say 75 minutes. This will be followed by 45 minutes as the next run which is then followed by a mix of tempo and easy totalling 46 minutes then the next weekend is 90 minutes and that is followed by two 30 minut runs and so on.

I'm sure there is some science behind it, can anybody explain?

Thanks

Ray