I see a lot of online videos with more professionals and experienced pilots using the iPhone to control the drone, not the iPad. Just wondering why and if theres benefit in doing that?

None really, just a matter of what people have on hand. I did not want to use my phone, so I bought an iPad because of the larger screen.

If you have an iPhone but not an iPad, you save money by not buying an iPad.  Using an iphone helps portability.

Downsides are smaller screen & people might by trying to call you all the time (not a problem for me, having no friends ).

You can buy an ipad that is wifi only that allows you to use all features except Follow Me.
lf l ever wanted to try Follow Me l'd use my iphone rather than pay extra for a cellular ipad.