No theorist claims to be prescribing a system of rules, any more than Newton can be accused of prescribing a set of laws by which nature must operate.

However, newtons observations were an essential step in helping us understand the mechanics of nature.

Music theory is the same thing. It is the amalgamation of our understanding about music based on observations and analytical thinking.

With a good understaanding of theory it is easy to handle unfamiliar or complex music.

Without it, unless you are a genuine geius (in which case you probably figured out a very similar set if noit the same set of observations) you will find much unfamiliar music and most complex music very difficult to understand.