Something I've been ruminating on for the last day or so:

Why is up used in so many expressions? We say that we "straightened up" a dirty room, the car "sped up", the slowpoke "caught up" with the rest of the group...

There are some expressions in which up makes sense to me, such as "I got up this morning" or "I picked up the dirty socks that were on the floor." These actually involve an upwards motion, unlike the expressions in the last paragraph.

Any thoughts? Or other expressions which I didn't come up with?