Exchanges and wallets ask for a phone number at signup, and it becomes a permanent handle on an account tied to money. Phone numbers are also the weakest link in that account: a SIM swap turns SMS recovery into an attacker's route in. A rented number clears the signup check without adding your personal line to a database that is a standing target.
This is for people opening an account they intend to hold, who want the phone number off the risk surface. That is a sensible precaution taken by careful people, and the site treats it as one.
Do not leave SMS as the second factor
This matters more here than anywhere else on the site. Use the rented number for the signup check, then set up an authenticator app or a hardware key as the actual second factor and remove SMS recovery if the platform lets you. A rented number is not a line you keep, and an account whose recovery depends on a number you no longer hold is an account you can lose.
What this does not do
It does not bypass identity verification. Exchanges that require documents will still require them, and a phone number has nothing to do with that step. This covers the SMS check, not KYC.
Some service and country combinations require one-time identity verification on our side before they can be used, and you are told before you spend anything.
Binance 
Coinbase 
Cryptocom 
Paxful 
Bitso 
United States 
United Kingdom 
Germany 
Turkey 
Brazil