Usually when redeploying new code or introducing critical changes to the server environment. It is common to restrict access to end-users for avoiding errors caused by end-users using an invalid or incorrect environment state.
Magento allows developers to set maintenance mode, redirecting all users to the application 503 page.
Also, the developers have the advantage of whitelisting one or more IP addresses, allowing them to test any change in the server without being worried about any end-user accessing the server at the same the server is being updated.
For example, the following command will enable maintenance mode for all end users except for 192.0.2.20 and 192.0.2.21:
$bin/magento maintenance:enable –ip=192.0.2.20 –ip=192.0.2.21
Enabled maintenance mode
Set exempt IP-addresses: 192.0.2.20, 192.0.2.21
The command above hopefully will be a helpful tool in order to reduce server downtime, the same for maintenance and development time.