Someone who knows the fat on them will chime in. I've seen several ..some with and without vacuum pumps. Essentially it appears to keep the crankcase void of any interference from containing oil. It has an external oil pump with various suction and pressure ports. Some scavenge any oil that drops into the sump.
The dry sump system usually had a pretty big oil tank compared to the oil pan capacity of the wet sump system. Often three or more times as much oil.
I suspect all of this kept oil cooler (at a time when oil quality was generally worse than today), avoided in the engine splashing and foaming, oil pick-up problems in cornering, and provided a good oil reservoir if oil consumption/leakage during a race was an issue.
I owned a 1961 Norton motorcycle that was dry sump, and a 1972 Honda 4 cylinder 750 cc motorcycle that was also dry sump.