> Surely rainfall is even over a surface regardless of terrain and therefore a ridged roof would collect the same amount as a smooth slopes roof?

Of course. It's the cross sectional area of the roof that matters, not the surface area. Imagine that there is no wind, and the rain is falling straight down. It's not going to make any difference whether the rain falls on a flat roof, a tall A-frame roof, a mansard roof, a hip roof, a roof with ridges, or a smooth roof.

Bermuda is primarily a limestone island. The original roofs on houses were made from limestone slabs quarried on the spot. The pit that the limestone slabs were quarried from became the cistern in which the rainwater was collected. Being somewhat thick, these limestone slabs gave the roof a stepped appearance. Over the years, this stepped appearance became the norm for what roofs were supposed to look like. Modern Bermuda roof systems are made to look like the old limestone tile systems because that's what people expect.

Long story short, the ridges are aesthetic, not functional.

P.S. Bermuda needs to collect rainwater because there are very few other sources of fresh water on the island. The government mandates that at least 80% of the roof needs to be dedicated to harvesting rainwater.