The first reason to reserve usernames is security. System terms like admin, root, and api look official. When a bad actor grabs one, your users cannot tell the difference between a legitimate account and a scammer. The same goes for support, help, and billing — these handles get weaponized for social engineering attacks.
Second is legal protection. If someone registers @nike on your platform and starts selling counterfeit goods, you are now involved in a trademark dispute. Platforms like Twitter and Instagram learned this early and built reservation systems specifically to avoid liability.
Third is user trust. When your users see @official or @verified accounts that are not actually official, they lose confidence in your platform. Consistency matters — if your support team is @company_support but someone else has @support, users will message the wrong account.
Fourth is future-proofing. Early platforms gave away handles like @ai, @crypto, and @nft for free. Now those single-word usernames are worth tens of thousands of dollars. Reserving premium handles early means you can monetize them later or use them for partnerships.
Finally, it is just good engineering. Usernames like null, undefined, and true break systems in unexpected ways. Admin panels crash. API routes conflict. Edge cases that seemed theoretical become production outages. Reserve these from the start.