I think Nicael has the answer correct, but missed a clue and interpreted another incorrectly.
So I agree it's
I can save a shift:
It does "save a shift" when active - you don't need to hold the shift to type the capital letter. Or, in a more direct meaning, if the shift button is broken, caps lock will provide you with ability to type a capital letter :)
I am not often used for my original purpose.
The original purpose of all caps was to communicate the entire text of what you wanted to say as clearly as possible, because lower case letters didn't exist.
Nowadays I'm mostly used for _________.
Read: "Nowadays I'm mostly used for underlining". Underlining is another way to emphasise words, which is what caps lock is used for now, NOT shouting (a small but vocal minority aside)