For now, I'll try explaining: the way I think of it is, it lets you do the exact same thing as macros, except live. Macros in vim/emacs/others are often touted as a great feature, and they are. In vim for example, formatting a huge column of, say, html text involves formatting one row using the proper keyboard commands, then repeating the action multiple times. But with multiple selection, you can operate on all the rows at once, easily seeing what it is you're doing and how it's affecting all the rows.
Maybe even more importantly, it makes it much easier to do activities you wouldn't dream of doing in a macro, since it's much easier and more light-weight. For example, say I have two very similar if statements, one which deals with case "x" and one with case "y". Say I wanted to add a small piece of code to both. I can, with one command, select both "if"s, then just write away that piece of code. Lets say each "if" has a different variable name, I can actually move the cursor up to the variable name, select the name, then paste it every time I need to use it. This is the kind of repetitive task that happens a lot in real-life code, but which you would never use a macro for since it takes to long to make work. With multiple cursors, it works easily.
Summary: multiple cursors make macro-like things 20 times more intuitive and much easier in practice, meaning you use them more, meaning you do less repetitive stuff.
Hope that helps explain it; for more details, you'll have to wait/check out Sublime yourself.