This is a bit of a style preference since of course you can always write the same thing with a for loop. However, I think sometimes the code looks better when using this method. It also provides an alternative to Sequence.map for cases where you don't need the resulting list, and one that becomes especially necessary when Sequence.map is changed to return a new sequence. The example in the README.md file was using Sequence.map in a way that required this alternative in that case.
The Wren scripts in this directory get converted to C string literals and then inserted into their respective .c files so that the interpreter can load them directly without having to do any file IO.
The script that does this copying is script/generate_builtins.py.
You can invoke using make builtin.