Do not really understand your question but let me try to answer it.

If you want to advertise using adwords, then you want to look for low competition keywords.
If you are writing blogs to earn money from adsense, then you want to target the high competition keywords.

For SEO ranking, you look for keywords with low competition, so that it's easier to rank for the keywords you want to rank for...

Low competition with long tail phrase is the best

Yep that's how it works.