Yes, you can work on your site without being in Maintenance Mode. Maintenance Mode could be used in these 3 scenarios.
1. You are making major design changes to your website and you do not want visitors to see your website until those design changes are completed.
2. There is a coding problem on your website and it is not displaying correctly to visitors. You can take your site Offline while you fix the problem.
3. The worst case scenario – your website has been hacked so you immeditately put your site in Maintenance Mode to prevent external Browser access to your website/remote script execution while you scramble to change all of your passwords: FTP, WordPress Login and DB password.
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