So why exactly do you have to use both and why in that order? If the jointer takes material off the bottom making it flat and straight, why couldn't you flip the board over and do the same to the other side?
A jointer makes a face flat, and there's no reason you couldn't make all 4 faces on a board flat using a jointer. The problem with that is that while the faces may be flat, you won't have a nice square board. Odds are, you'll end up with a triangle.
A thickness planer will also make a face flat, but the difference is in how it does it. A jointer uses an offset set of tables across a blade to shave off the high point on a piece of wood, just like a hand plane. A thickness planer, however, has a blade mounted a set distance above a table, to that whatever is run under it against the table will have a consistent thickness across the board equal to the distance of the blade to the table.
In short, a jointer flattens a face, a planer creates a second face on the board parallel to the first, as well as making the board a consistent thickness.
I'm horrible at explaining things. If you're still confused, The Wood Whisperer has an excellent video explaining the process of milling boards from raw lumber that breaks it down better than I can, I'd recommend checking it out