code loading docs: note the purpose of each kind of environment

doc/src/manual/code-loading.md

@@ -33,6 +33,12 @@ An *environment* determines what `import X` and `using X` mean in various code c
 2. **A package directory** is a directory containing the source trees of a set of packages as subdirectories. This kind of environment was the only kind that existed in Julia 0.6 and earlier. If `X` is a subdirectory of a package directory and `X/src/X.jl` exists, then the package `X` is available in the package directory environment and `X/src/X.jl` is the source file by which it is loaded.
 3. **A stacked environment** is an ordered set of project environments and package directories, overlaid to make a single composite environment in which all the packages available in its constituent environments are available. Julia's load path is a stacked environment, for example.
 
+These three kinds of environment each serve a different purpose:
+
+* Project environments provide *reproducibility*: by checking a project environment into version control—i.e. a git repository—along with the rest of the project's source code, you can reproduce the exact state of the project _and_ all of its dependencies since the manifest file captures the exact version of every dependency and can be rematerialized easily.
+* Package directories provide low-overhead *convenience* when a project environment would be overkill: are handy when you have a set of packages and just want to put them somewhere and use them as they are without having to create and maintain a project environment for them.
+* Stacked environments allow for using tools during development that aren't dependencies of the project you're developing: you can augment your primary environment—i.e. the first environment in the stack—with additional environments containing tools and utilities that you want use interactively.
+
 As an abstraction, an environment provides three maps: `roots`, `graph` and `paths`. When resolving the meaning of `import X`, `roots` and `graph` are used to determine the identity of `X` and answer the question *"what is `X`?"*, while the `paths` map is used to locate the source code of `X` and answer the question *"where is `X`?"* The specific roles of the three maps are:
 
 - **roots:** `name::Symbol` ⟶ `uuid::UUID`