Why use master to create a database?

It is absolutely not a requirement in this very specific case, but it is a requirement in many other scenarios. If you're creating a database called Sales, and you arelady have a database called Sales, you'll need to change your database context before you:

• Restore with replace; or,
• Drop the current database and then:
  • Create from scratch; or,
  • Create for attach.

There are plenty of other scenarios outside of database creation that also require either (a) not being in the context of the current database, or (b) being in the context of master specifically (or at least not a specific database), and many of these things you may be doing during or around creating databases:

• Setting a database to a different state, like single_user
• Preventing errors when a script has a USE command but that user database may be offline or otherwise inaccessible
• Granting server-level permissions like CREATE DATABASE
• Granting server-level role membership
• Marking a module as a system object (sp_MS_marksystemobject) or as a startup procedure
• Certain types of certificate, server audit, and Availability Group operations

Probably a slew of other things. USE master; isn't always necessary, but sometimes it is, and it doesn't hurt to always execute server-level commands from that database.

I believe it's not a requirement that you should use the master database to create a database. Since the create database command should be run in a database context the documentation always uses a default database which is master and it's a system database which will be always there no matter what so the script doesn't fail!

I believe that we, a very very long time ago, had to be in master when executing the CREATE DATABASE command. I'm too lazy to fire up some 20 years old version to verify. So probably a combination of that and that it "feels natural" to be in master. Like if you are to create a folder (imagine there are just one level of folders), you probably feel "better" to say CD \ first (comparing the root here with master database).