A major motivation is that it allows us to handle the problem of when it’s not safe to use 

[python-setup].resolve_all_constraints

 and the repository PEX feature. For example, w/ a 

pex_binary

 using 

platforms

, the subset of reqs might be valid, but the superset of all your reqs might include deps without wheels, so the superset fails to resolve. This was a real problem hit this month and now fixed to not use repository PEXes when platforms are used. But, we do still want to use a constraints file to lock down all versions

so, this is why i have discussed the templating of the platform and the interpreter_constraints into each resolve