There are really two different, substantial reasons for the existence of patterns.
The first has already been explained pretty well: the use of patterns lubricates communication between developers. If you and I both understand that when I say 'Observer' I am talking about a very specific structure of code, then I can very quickly describe how a bit of code which uses that pattern works. The alternative is to fully describe the solution, which is time consuming and error prone. ("Well, I created this pure virtual class which describes and interface for consumer objects, and then I created a class which maintains a list of active consumers, which ...")
The second benefit of patterns is that they are off-the-shelf solution-forms for common problem-forms. If you know your patterns, and, for example, you encounter a problem where you need to find a good way to get information from (possibly multiple) producer objects to multiple consumer objects, without introducing unnecessary coupling between classes, you will recognize "this is a job for an Observer!" and you will immediately know how to solve your problem.
These benefits also really reinforce each other. They allow you to quickly solve certain common classes of problem, and then when you're done, you can very quickly communicate how you solved the problem.
Contrast this with a world where patterns "don't exist". You run into one of these classes of problems, which are generally not trivial design problems, and you spend a fair bit of time coming up with a good solution (which, incidentally, will very likely look a lot like the appropriate pattern). Then, your co-worker comes up and wantx to know how you solved it, and you spend an hour discussing the how and the why.
This is all associated with a caveat which should seem pretty obvious: don't try to force problems into patterns that don't fit. If the pattern doesn't fit the problem, then the solution will end up being convoluted and you will lose the effort reduction benefit of patterns. Additionally, since your work will no longer fit with your co-workers' understanding of the meaning of the pattern, you will lose the cost of communication benefit. In fact, you will likely increase the cost of communication beyond the no-patterns cost, because the misuse of the pattern will give your co-workers a false understanding of the solution, which is worse than no understanding at all.