Answer:
You don't really need to do it, but it helps you keep things more organized and easier to follow. Imagine if you're doing some multi-variable equation,
2a + 5b + 4d + 3c + b + a + 2d
that looks like a mess, it'll be easier to look at if you put all the similar variables next to each others like this:
a + 2a + b + 5b + 3c + 2d + 4d
(a + 2a) + (b + 5b) + 3c + (2d + 4d)
now you can add them up much easier.
