To counter the inevitable erosion, the electrode must be made large, and that means more of the combustion chamber is taken up by the electrode. Platinum is equivalent to traditional materials in its conductivity, but has a much higher melting point and resists erosion much better. The platinum center electrode can be made much smaller, which promotes a stronger, smaller spark. The smaller center electrode allows more room for the flame front to propogate out away from the plug. Platinum plugs are, of course, much more expensive than traditional spark plugs.
Iridium is the king of spark plug metals.
It has strength and melting temperature far higher than even platinum. The longevity of these plugs is unsurpassed. But no plug will last longer than an iridium one.