One of the tenets of good security is that no person or process is granted more rights than it needs to accomplish a task. This is the Principle of Least Privilege, and is designed to ensure that your system is as secure, stable, and predictable as possible.
For a SQL Server instance, this means that unless a person or process needs to perform instance level changes (configuration, patching, etc.), then they should not have instance level privileges. If a process accesses data in a database, there is no need to grant sysadmin privileges, even if they do need db_owner privileges.
Many software packages require sysadmin access be granted, often without any justification. Developers build software as a sysadmin on their own workstation, or on a development server. They often don’t understand or realize that less privileges will still allow their software to work. As a result we find software running under elevated privileges, which can be a security risk if there are bugs or injection holes in the software.
It’s hard to make changes later on, and it can be frustrating to argue with vendors that their software doesn’t need elevated privileges. The best way to combat this is to educate yourself and any developers you know about security.
Adding login or user accounts does not require sa privileges. The securityadmin and db_securityadmin roles can be used. The rights to create objects in a database can be handled with granular permissions in a schema, or with the db_ddladmin role instead of db_owner.
Work with limited rights when you can, and help other learn how to work with the minimum they need and we’ll all be more secure.