What’s the deal with the Gundam in the Z’Gok?

I’m mostly a UC fan and I’ve never watched Seed, Destiny, or Freedom. But as a Gundam fan online and on Reddit, I’ve certainly absorbed some broad strokes about Seed.

That being said, the whole Gundam in a Z’Gok is so weird to me… Why does it make sense in universe to stick a Gundam in another suit? It seemed like it was kinda fan servicey, like an Easter egg or something. Why does the fanbase care? Why is it a thing?

I realize the answers to this are probably spoilery. For me, I don’t really care. I’d rather be spoiled and have my curiosity satisfied!

Thanks!