For people coding in plain C Glade and GtkBuilder can be a big help indeed., as plain C is so verbose that making changes or only understanding code some time later can be very hard.
For the modern high level languages like Python, Nim, maybe also Vala, Rust, Go, D and many more I can not see such a big benefit for using Glade and GtkBuilder.
I started using Glade some years ago, at that time Glade documentation and and tutorials was really not good, and GTK community was not very active at all. I can remember struggling with a point, sending message to gtk mailing list:
Of course no reply. My NEd Nim editor created at that time is using glade, and I added some GTkBuilder examples to the Nim bindings page, but never used Glade myself since then. Maybe documentation and tutorials have improved, so I may give it a try again at some time.
Generally GtkBuilder like QML https://en.wikipedia.org/wiki/QML may be a good solution for fast prototyping of user interface–centric applications in a declarative way.