Finding a bunch of keywords that you want to rank for is easy. The hard part is figuring out what it’ll take to rank for those keywords (i.e., “keyword difficulty”), and then using that information to prioritize and plan your SEO strategy.
Nobody knows the exact formula Google uses to rank web pages. Even if they did, it wouldn’t change the fact that some keywords are more difficult to rank for than others. That’s what makes the concept of “keyword difficulty” one of the key challenges in SEO.