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.