Just a bigger gap (hence higher voltage spark from the increased resistance), and more surface spark effect.
In some engines, this will make a difference. Hans tells me they used to open up the gap to almost a full 1 mm in one of the 60's sixes (don't remember which one) and that gave much better idle and off line performance. Only worked in the one, in either of the other two common ones (230, 250, 280), the opposite was true, it ruined the idle.
All depends on the engine, I suppose, just like some engine run better on a certain brand of spark plug, or are greatly improved by indexing the plugs while other it doesn't matter.
I'm perfectly happy with the Bosch Platinums (single electrode, straight non-resistor wireset) in the 280. My problem there is keeping the points clean, I suppose I need to de-oil the upper part of the distributor.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!