When should you use Keywords?
When you've got your keywords or key phrases... you should look to include them in the first 150 words of your page or content.
By doing this, Google thinks... hey... there's a Title about That thing there, oh wow, and they're talking about it underneath that title - they're probably talking about that thing.
So by having your URL, Page Title, Headings, Content all using keywords, and early on, Google (and other search engines!) will quickly think that you're probably talking about that thing.
Remember... if it walks like a duck... talks like a duck.... looks like a duck... it's probably a duck.