Hello,
The most common, is the one pointed by Zaid Tariq, and it is mostly used to grant access to any database when, for instance, you want to restore a customer's database on a local consultant's PC, then, as normally the customer´s db will have permissions for certain users, then, if you do not perform that deletion, you will not be able to even create a Server Instance in order to connect to the db with the application.
However, I might say that for that purpose I use the foloowing:
USE [DatabaseName];
TRUNCATE TABLE "User Personalization";
TRUNCATE TABLE "User Property";
TRUNCATE TABLE "Access Control";
TRUNCATE TABLE "User";
GO
And, as others has said on previous answers, to use SQL to solve problems is no the recommended way, but it is a great way of review the data. Or, depending on your needs, update some of it. I have use it very carefuly and with success to work on filling related tables, lists, charts, and so on. But never, NEVER, use it to modify records that were posted, or that just can be posted in the application using a process, because, this will cause several problems and malfunctions to your database.
Anyway, If you need to review, copy, analyze, or even update any information, I share with you this site, where I found and use many SQL commands, with good examples of how-to-use.
https://www.w3schools.com/sql/default.asp
Hope the information be useful.
Regards.