In case anyone doesn't know what this is. GI is a system for accompanying system components (i.e. shared libraries) with XML meta-data (in separate files) that describes their API's: types, functions, arguments, and argument semantics (in/out pointers: who owns/allocates/frees, and whatnot).
Here is the significance: if the GI info for a component is correct, and if you can parse it in your language, from that info you can generate accurate FFI bindings, without manual work.
The accurate, detailed semantic information cannot be gleaned even if you parse the C headers. Without something like GObject Introspection, you're looking at a lot of manual work.