The word "female," in its primary usage, is an adjective. When you use "female" as a noun, the subject that you're referring to is erased.
"I talked to a female yesterday."
A female what? A female kangaroo? A female rock snake? The subject of the sentence is not clear.
"I talked to a female presidential candidate yesterday."
This sentence is now about a human being.
It should be noted, though, that using "female" as an adjective can take a sexist turn when used in a case that isn't notable. Referencing a "female firefighter," for example, is appropriate only when her being female is pertinent to the story; otherwise, she's just a firefighter. But if you're talking about the first woman to become a firefighter, saying "the first female firefighter" is acceptable because her gender is relevant.
For more information on the appropriate grammatical uses of "female" and "woman," click here.