What happens in instances of SQL Server users that make them impossible in non-express versions?

Question

After developing for several years (or more), I still don't understand:

• What makes user instances impossible / incompatible with the version of SQL Server (or any other)?
  • Why should a developer install more than one (for example, a developer version) SQL Server because user instances are only available through Express Edition?

Update: @Damien_The_Unbeliever wrote in his answer

User instances are pretty well expected as standalone databases used by only one application, and without using this application are meaningless.

Do I understand correctly that user instances:

• SQL Server Express installation required
• cannot connect remotely
• cannot a user have more than one such instance?
• are the client applications that connect to them such tools as VS, SSMS, webadmin, SQL Express utility , etc.?

Indeed, I do not understand when / why / how they can be used / deployed outside the development environment

What are the non-dev SQL Server User instance files (vs CE and other embedded databases)?

Answer 1

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