Being ourselves

Generally we hear this phrase a lot of times, "Be yourself".
It used to me make me wonder that why do people advise others to be yourself.
What else can people be other than being themselves.

But sometimes, yes, people do and say things just to impress people though it is not themselves.
Either to impress, patronize others, or expecting some favor in return, reason could be anything.

How much are we ourselves on a day to day basis?
Some random silly thought on a weekend night.