Lids prevent spilling half your morning green smoothie all over your car and your pants when you're kinda running late to work. And, just as an example, it could very well make a mess very similar to this:
But, on the bright side, with the exception of spilling one all over myself, those green smoothies make my mind extra sharp. That's how I was able to crush the high score in Ms. PacMan at the laundromat this weekend. 84,920. Oh yeah.