I don't think I have a good answer for that question! I sell about 20'- 40' of 4" round solid Aluminum a year. But I can't recall asking one customer what they're using it for.
I do have one customer that machines parts out of various aluminum profiles. He went to school in Florida. But usually ships his parts from Canada state side. As far as I know his customers are in the aerospace field.
The largest aluminum round I have used is 6 in. Years ago when I made this coffee table photoed below. Today I might cut circles out of plate. But back then, I got some 6-in solid cut into these plates that are above the glass on the table.
I guess if you were using a lathe round would be a good starting point.
I was thinking the same thing you were, slice off an end into rounds, but figured that couldn't be the most popular use.
I bet there is a part on NASCAR's stock cars that starts as 4" aluminum round solid, and is turned on a lathe to it finally shape. Just a thought!
The sales page said these were the uses, but I never did find any projects using it.