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Consider this scenario, the Account entity has a Sensitive Data field which holds 'Top Secret' information.
But look what happens when the user decides to use the built in developer tools (F12) in their browser. They are able to see the 'Top Secret' information.
If you need to have data on a form that is accessible to some users and not to others using Field Level Security is a better solution.
Now when the user tries see the 'Sensitive Data' they don't see anything other than the masked value.
If you want more information on security in Dynamics CRM, check out these links: