Lyle's Golden Syrup tin design, why is it like this and how to clean it
I suspect you are looking for deep explanations where none exist.

Flat metal work is flexible; by introducing a bend, it becomes stronger.  This is the same principal that makes corrugation, and the same reason car parts are all curved.  You will note that paint cans have essentially the same design, for the same reason.

The lip around the removable lid is quite likely to be there simply to give it enough strength to allow the lid to be removed and replaced multiple times without distorting the can.

This has nothing to do with the specific product inside.

As to cleaning, a damp paper towel seems ideal.  If the lid is well sealed, you could run it under some water to dissolve any caked on syrup, and then dry...