pure Nitrogen in tyres leads to a more predictable pressure gradient as the tyre heats up. It is used in the tires on aircraft landing gear for this reason, and also in formula 1 racing car tyres. Both usages place incredible stress & strain on the tires.
A side effect is that tires with pure nitrogen lose pressure are slowly, as it is harder for pure nitrogen to escape - no doubt a chemist could explain why. My guess the molecules are larger than regular air molecules, but I am guessing. Think there is a documented benefit for regular bike and car tires when running at altitude, in that pressure loss is slower for this reason.