Nitrogen inflation prevents oxygen from degrading the rubber. The inside of a car tire is higher pressure than the outside and gets warm from rolling resistance induced friction so oxidation is worse from the inside.

Tubulars don't last long enough for it to make a difference.

Maybe nitrogen lasts longer in some kinds of tires but as someone has pointed out, if it lasted longer inside a racing tubular then you would only ever lose 20% of your pressure and only on the first inflation. On the second top up, you could only lose four percent. On the third top up, less than one percent.