What is the bowl in the middle for?

The central glass vessel served as a mixing bowl. Tea was kept in the two side compartments, usually green in one and black in the other. The host would scoop leaves from both sides into the bowl to blend them before brewing. Sometimes these bowls also doubled as sugar basins. It depends on the specific date of the piece and the household habits of the time.

My grandmother left me this heavy wooden box with a curved lid, but there are no keys and I cannot get the main lid open to see inside, so how do I unlock it?

A local locksmith can usually open the box without damaging the wood or the brass escutcheon. They use a set of skeleton keys or fine picking tools to gently turn the mechanism. Don't force the lid with a screwdriver or knife. The lock mechanisms are quite simple, but prying the wood will crack the veneer and ruin the edge profile permanently.

How do you clean the inside?

You just need a soft bristle brush to sweep out any old dust. A camera lens brush or a clean paintbrush works well for getting into the tight corners. Never use damp cloths or liquid cleaners on the interior. Any moisture can lift the remaining foil lining or warp the thin wooden dividers.

I have been looking at different caddies online and I noticed that some of the earlier wooden ones seem to have metal handles on the top, but the later ones never have them, why did that change?

The overall design shifted from tall, single-canister styles to wider, heavier boxes. Early single caddies weren't too heavy, meaning they could be picked up by a small top handle and passed around. By the early nineteenth century, the sarcophagus shape became dominant. These boxes held two heavy lead-lined compartments and a thick glass bowl, making them too heavy for a delicate lid handle. Makers fitted sturdy cast metal ring handles to the sides of the carcase so servants could move the box safely.