Exclusivity is a common add-on to a brand deal rather than a default term. Without it, a creator can work with a competing brand the next day. With it, a brand pays extra to lock the creator's category out of its own competitors, which protects the brand from a follower seeing the same face pitch two rival products in one week.
The strength of the clause depends on how tightly it is written. "No skincare brands for 60 days" is enforceable and easy to check. "No competitors" with no named list or end date leaves the creator guessing which future deals are off limits and for how long, and that ambiguity should be negotiated out before signing.
On Socialync's job board, any exclusivity requirement is stated in the campaign terms before a creator applies, the same way pay and usage rights are, so it is a known cost of taking the job rather than a term discovered afterward.