What is the purpose of a brand?

I agree with other answers but in addition, a brand's "purpose" (from the vendor side) is to increase profits.

The value of a brand is equal to the delta of what a product is 'worth' and what people actually pay for it. Analysts have said that 2/3 of Apple's market cap is their brand equity, not physical assets like cash or IP.

Needless to say many brands don't actually achieve their purpose-- they are boo'd at, mocked, and defamed by sensible consumers with keen eyes for bullshit tactics and poor ethics. But the purpose remains the same-- more money.