Ya know, nothing annoys me more than people who can't read, so let me help you out with that.
It should be noted, that the env vars would only be set by people installing spring via the spring installer.
Portable, being what it is, there is nothing to be gained by setting env vars for those installs as the location might change at any given moment.
You don't let the user choose what directory spring/evorts is installed in? That alone would cause huge amounts of grief on my part.
Desura (just like steam) gives you the option of where to install.
Whenever evo is started, an env is set to point to evo's install location. That way, no matter where it is, any time evo is run, it will point to it's install location (usually (But not always) c:/program files(x86)/desura/common/evolution rts).
The main reason I did this is so I can do clever shit with error reporting (and other things too, just error reporting always ends up being the most common). For example, when I tell someone to go poking around for their evo install, I can just so open up windows explorer and where it says "My Documents, erase that shit and put %EVOLUTIONRTS% and press enter.
VIOLA!!! Evo install folder. Now gimmie the infolog.
Or later, I can write a program that automatically uploads an infolog to my server when a user does something or another. The uses of the env var are to make it very easy on game devs. I do not need it for myself, but for the rest of the gamedevs, it could help make their lives much easier.