If it is not going to actively heated or cooled, is there any value in insulating a garage?
Oh, yes

It's a cornerstone of passive solar design.  

We spent $2.4M on a large building we intend to never heat.  We insulated it to the nines.  Here's what that does. 

Extremes of temperature are moderated by building's thermal mass*, which (with the insulation) helps it resist changes in temperature. This affects

Condensation, which is significantly reduced or eliminated.  This makes life much easier for the contents of the building.  

If you don't have enough thermal mass*, you can add some by installing large barrels of water or antifreeze. 



* ”thermal mass" is the terminology used in building design, but it's actually a misnomer. Mass/weight does not store heat.  Atoms do.  And atoms vary in mass dramatically from 1 for hydrogen to 207 for lead.  But an atom's heat storage ability is about the same, 20-30 j/mol/degK, most 24-26 j/mol/degK.  Different materials contain different mixes of atoms, so their heat capacity by mass is all over the map.  Water is ideal.