Puncture repair

Whilst in Ramsgate I had a puncture, fortunately not whilst towing our caravan. Changed to the spare wheel and took the flat one into Kwikfit where I received excellent service from the staff. Now the repair came to around £26.50. £13.50 of which was for the repair. £6.75 to balance. £4.75 for a new valve and £1.50 to inflate the tyre with Nitrogen, for which the receipt explains all the benefits.But I have to ask, what is the benefit of having only one tyre inflated in this way and where do I continue inflating the tyre with said gas?