I'd like you to take a look at your yarn stash and read some labels. Quite often when we start a project we just discard the labels or leave then to languish in the bottom of the project bag, but they have a lot to say.
What the yarn is made of: The more you knit the more you learn about how different fibres behave so the fibre content listed can give you clues about how the yarn will be to knit with and what sort of fabric it might make.
Recommended needle size and tension: This doesn't mean that you must use these needles or that this is the tension that you will get in a particular pattern. Rather it is the average tension the manufacturer has found for that yarn on that size of needle. But this does give you clues to what range of needles this yarn will work best with.
The amount and length of the yarn in the ball or skein: Labels will often tell you how many metres there are in the ball as well as the weight. Some patterns will tell you how many metres of yarns are used in a project, so this can be very useful.
Washing instructions: Very useful - if I am giving a knitted item to a non-knitter I will often include a yarn label so they have official washing instructions.