Many users would like to use environment variables in Jamfiles, for example, to control location of external libraries. In many cases you better declare those external libraries in the site-config.jam file, as documented in the recipes section. However, if the users already have the environment variables set up, it's not convenient to ask them to set up site-config.jam files as well, and using environment variables might be reasonable.
In Boost.Build V2, each Jamfile is a separate namespace, and the variables defined in environment is imported into the global namespace. Therefore, to access environment variable from Jamfile, you'd need the following code:
import modules ; local SOME_LIBRARY_PATH = [ modules.peek : SOME_LIBRARY_PATH ] ; exe a : a.cpp : <include>$(SOME_LIBRARY_PATH) ;