What are the advantages of filling tires with Nitrogen instead of air? I am completely new to riding bikes and know that are usually filled with air but nowadays some people fill them with Nitrogen instead.

I don't know why they prefer to fill their tires with nitrogen as it is more costly than filling them with air.

What are the benefits of filling tires with Nitrogen?
There are some compelling reasons for filling vehicle tires with pure nitrogen in performance situations, but for bike tires it's just snake oil. There aren't substantial enough temperature fluctuations in a bike tire to justify the pressure consistency argument. 

It's also worth noting that the air you breathe and fill your tires with is more than three-quarters Nitrogen, so this isn't as special as it may sound.