Making intentions known is necessary. Flirting is just one of the preocesses by which intention is established. In that way, it is not strictly necessary, in the same way that cake is not necessary, but food is.

Flirting is a process which intentionally leaves ambiguity because it lets people avoid embarrassment. Being rejected, in many cultures, is embarrassing. By attempting flirting, a person can show interest indirectly, and the other person can show interest in return or show disinterest with quiet cues that let the instigator pull back without having to do something as vulnerable as explicitly stating intentions or experience embarrassment at being directly rejected.

Flirting, like anything else, can also be used as a display of quality, the verbal/intellectual equivalent of peacock feather displays.

How necessary these elements are is entirely contextual. Some partners despise the pretense of it. Others view it as incredibly fun. Some are deeply embarassed by the prospect of rejection. Some are not bothered by it at all.