This question came up yesterday while talking to a friend that did not know that you use different sized sewing needles for different weight fabrics. They wanted to know why their sewing machine was jamming and breaking the needle when trying to sew jeans.
Here is my simple needle guide
Sheer to lightweight = 9, 11
Lightweight = 11
Medium-weight = 14
Medium to Heavy-weight = 16, 18
Denim/Canvas = 16
Lightweight Knits = 10, 12
Medium to Heavy-weight Knits = 14
Leather/Suede = 14, 16
Truth is you can use one size needle but the results will not be what you expect or even worse. Too small a needle used on jeans or leather can break and potentially harm you or your machine. Too large a needle will ruin your fabric or snag it making unsightly runs in the weave. Another thing that can cause poor stitching is a dull or bent needle on the fabric your using. It is best to use the right sized needle or wait until you have it to start a project, that is what I do.