Imagine doing those manually instead of using the shortcuts, every time. Now are you getting a sense of how much time those little keystrokes save you?
How to use the Alt key (a.k.a. the "magical shortcut to darn near everything" key)
Hold the Alt button.
Notice that this does two things:
1) causes certain letters in your menu items to be underlined, like so:
2) causes numbers to appear over your first nine action bar buttons.
While holding the Alt key, you can press one of those numbers or underlined letters. For example, holding Alt and pressing T will open the Text menu, like so:
See how the k in "Strikethrough" is underlined above? That means that pressing K will activate the Strikethrough formatting.
To recap that example:
You get this: Stricken-through text
Try it! (To turn off strikethrough, just press Alt+T+K again).
The secret Master Shortcut Formula
To access any function on any menu, just hold the Alt key and press the underlined letter of the thing you want to do. This also works with any numbers that pop up when you hold Alt. Go ahead and explore -- find the shortcuts for your favorite functions.
Hint: Alt+W is really handy.