As developer or Admin when developing complex stuff, configuring the security model, debugging user support.
We should able to login as another user in Salesforce which will make sure that user only have access to what we have actually configured or the security model is correct.
In order to enable this feature
Go to Setup --> Security controls --> Login Access policies
Enable Administrators Can Log in as Any Use checkbox.
Now go to Setup --> Mange users --> All users.
You able to see the login link.
This is a simple one but I am sure a super useful feature to have.
Have a great day !!!