Patterns tell you specifically what you need to do to achieve a certain result. You may know how to make a crochet sphere, but can you automatically figure out how to change the sphere sizes to make a teddy bear? And are the teddy's arms the same, or is one a little fatter because you forgot precisely how many stitches you had started with on the first one.
You may be able to plan out the dimensions needed for a scarf, a sweater, or what-not, but do you really want to go through all the trouble of figuring out how many stitches you need where, how you are going to shape it, and how you want the fabric to look when you are done?
This mess is a pain to begin with, but it is a particular problem with three-dimensional items. That is why we have patterns: so we know precisely what we are making, how to make it, how much yarn we will need, and tips to bear in mind while making it.