[dba-SQLServer] Previous versions of SQL

This is a very general question, I realize, and although I still have
numerous gigs available, why do I need versions of SQL before SQL Server
and Visual Studio? Since I began my retirement, I retain an interest in
these technologies, but have no current clients to support, so am I free to
do a backup of everything and then reformat the drive, re-install Windows 7
and then only the most current parts? The reason I ask is that I still want
various 3p products to work, such as the Red Gate stuff for SQL Server. I'm
guessing that such things will automatically install what they need, but am
unsure about this.

I want to pare down my environment to the minimal set of stuff that I
actually use, I have a list of these components ready. Why would I need SQL
2005 for example? All I'm interested in currently is SQL 2012; do I still
need an instance of SQL 2008, SQL 2008 R2, SQL 2005 etc.? And why do I have
so many instances of various versions of Visua Studio run-times?

I'll back everything up before I flush the proverbial toilet, but why must
I have all these instances?

-- 
Arthur
Cell: 647.710.1314

Prediction is difficult, especially of the future.
  -- Niels Bohr