When writing a equation, do you simplify it?
For example, each pencil cost $2 and a eraser cost $4, but each order has a 3 dollar fee. (if you buy p pencils, e erasers) Do you do 2p+4e+3(a+e), or can you simply do 3p+7e? if you substitute the variables for any number you get the same answer, this is what i mean by simplifying it.
once you have your expression, you can always use it as-is, but life is generally easier if you simplify, as you did above.
If you have to substitute in values for p and e, and evaluate the expression, why do more work than necessary?
Also, most teachers pretty much insist that you simplify such expressions. It's just good practice.