We all do things without knowing exactly why but I'd like to know the answer to this one;

When building a model, if you duplicate your "half" model to become the other half (like in the dinosaur tutorial) then COMBINE the original half and the copy, why do you have to MERGE vertices from the 2 halves? Is it a must or is it just something you have to do if you plan on converting to SubDs?

Also,
I noticed that if you combine 2 poly objects, they keep their own UVs and material. When I made my dinosaur model this worked well for the inside of its mouth. I didn't have to worry about trying to paint each of the model's teeth. The teeth were grouped together then combined with the gums model.. The gums model had one texture and the teeth had another.. The question is, what if I didn't want them to have seperate UVs and materials. Would I have to Either Boolean them together or merge their vertices/edges? Can someone explain this?