Voice is personality
Your brand is a person: it has a tone, a vocabulary and a way of saying things. Serious or playful? Formal or close? Brief or detailed? Those answers are your brand voice, and they must stay consistent across every post.
Why it matters
Consistency builds familiarity, and familiarity builds trust. When your audience reads a line and says "that is them" before seeing the name, you have a real brand, not just an account.
How to build it
Pick 3 traits for your voice (say: confident, direct, informal), write "we say" and "we never say" examples, and share them with everyone who writes for you. A one-page guide aligns ten people.
They know you from the first line, without seeing the logo.