There are only 3 things I can think of to do with metal progression. 1. The main one. The better the tools the faster you can gather materials for creative builds.
2. Equip some nice gear and go fight some enemies for fun.
3. Create a structure out of metal blocks as a monument of your progress (needs insane amounts of metal per block - 12 ingots, so you'd need not only a lot of metal, but also a good smithy to mass produce plates)
Basically metal progressions can improve things, but the fun in the game doesn't really rely on metal progression, cause all of the features are basically available from copper age, and better metal only enhances them a bit instead of bringing something new.