UML emerged out of the whole CASE (Computer Aided Software Engineering) movement. (Now there's a term you don't hear much any more!) Things were never well standardized and UML was an attempt to bring some sanity to the whole undertaking.

I used to work on a CASE tool early in my career. I was always struck by the fact that we never ate our own dog food; that is, we never employed the tool we were working on to guide its own construction. This was the first time my eyes were opened to the fact that perhaps there was a lot of junk being built :)

CASE itself I believe was an attempt to replicate for assembling bits some of the benefits CAD/CAM was bringing to assembling atoms.