Do you need cellular data on iPad Air 2?

You would only need the cellular if you wanted to access the internet when out of range of a WiFi connection.

And even then you don't have to have a cellular iPad. There are devices like the Mifi, which is a portable wireless hotspot you connect your iPad to.

However, I've had a wifi only iPad for years and have gotten along fine using various hotspots. Ultimately it comes down to how you need to use it and how available wifi is.