I would say that installing the “full” version of SQL Server Development Edition makes sense if you are seriously developing the database.
However, you can install both side by side, and SQL Server Express is very useful for one reason - it supports placing database files in the App_Data application's App_Data folder. Although I do not do this for my own projects, it is a very nice feature for demos or open source applications that you download from the Internet: just unzip, launch VS, press F5, and you have a working solution, including a database. Without SQL Express, you first need to move the MDF file, attach the database, configure user rights, replace connection strings, etc ...
realMarkusSchmidt
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