Even MS's own docs explain why there are two of them, thus why you'd want both.
Install SQL Server PowerShell module
There are two SQL Server PowerShell modules; SqlServer and SQLPS.
The SQLPS module is included with the SQL Server installation (for backwards compatibility), but is no longer being updated. The most up-to-date PowerShell module is the SqlServer module.
The SqlServer module contains updated versions of the cmdlets in SQLPS, and also includes new cmdlets to support the latest SQL features. Previous versions of the SqlServer module were included with SQL Server Management Studio (SSMS), but only with the 16.x versions of SSMS. To use PowerShell with SSMS 17.0 and later, the SqlServer module must be installed from the PowerShell Gallery.
RE: "Why would you feel there would be?" Who says I did think there would be? But I do know that at times there are such issues with having multiple versions of "same" "product" / "code" side by side and have learnt to be prudent.
RE: "Even MS's own docs explain why there are two of them, thus why you'd want both." That says a lot about you, your knowledge and your experience.