Without a reservation, tunnel services typically generate a random subdomain on each session: ngrok's free plan does this. The URL changes whenever the agent reconnects, breaking webhook integrations that pinned the previous URL.
Reserved subdomains solve this by letting you claim a name once and keep it. lrok includes one reservation on the free plan. ngrok requires Pro+ for reserved subdomains.
The practical effect: paste the reserved URL into Stripe / GitHub / Auth0 once and never re-paste. Your dev loop stops drifting daily.