Not attacking with all forces so you can deside afterwards where you put your main force?
Or leaving behind part of your forces
In some ways these are the same thing, as for me the answer to the question is rarely; or automatically.
“strategic reserves” deliberately left places are generally poor form; as an example the Japanese should always work to get their units off valueless islands; and redeploy them where they matter. Or AAA’s shouldn’t be left in US capitals, move that junk to the front and maximize your power!
Each piece should move each turn and have purpose. As Larry said in his 1998 Hasbro A&A Video Game intro, you need to move like a field marshall and hit your enemy with everything you’ve got. This is why I rarely ever leave anyone behind as a “reserve”.
That said, builds can give you what I would call “automatic” reserves. Example, Germany is under the pressure but needs units for Moscow. The units you build in Berlin serve a dual purpose of keeping pressure on the enemy, whilst also protecting your capital. Or for example, you can do the same thing in West germany, build 10 mech there that are destined to go to the east for Moscow, but should the allies land in france or rome, you can hammer them with that force if necessary.
Aircraft placement is generally the “best” kind of strategic reserve option, as where you choose to place your aircraft will determine what options exist for you the next turn, without tying needless resources into areas that can be avoided.
Rommel has a great quote in the movie Patton, where he gets upset with Hitler for not allowing the redeployment of tanks/armies. The quote is something like “Our men are throwing pebbles at each other on the beach in Calais, whilst our other boys are getting pounded in Normandy; I wish we could redeploy our forces, the allies would know they fought an army instead of a bunch of stationary targets!”
This quote holds some truth for A&A.
