Why do we call some people good specialists?

A good specialist is someone who knows a lot about one (maybe more) topics. For example, one girl knows a lot about flowers. She knows a history of every flower, how to grow them and what are advantageous and disadvantageous. She read a lot of books and practised her knowledge in real life, in the result she has a huge collection of unusual flowers. That is why we can call her a good specialist because we see a result of her knowledge. We can ask her about flowers and she will help us, so we wont need to waste time for searching and reading a lot of unuseful information if we just want to grow roses.