The iPad is no replacement for a MacOS, but there are a few things it has helped me with: first and foremost, sheet music. I use apps like ForScore and iGigBook and that is truly liberating, not having to schlep binders with every song I might conceivably need.
Mine is a 4th-gen cellular model with 128GB, and I have a little Logitech keyboard, which makes it a much more usable all-round device.
I mention the storage because 128GB on an iOS device is way more than 256GB is on a Mac. Mail on Mac easily eats up like 40GB on a Mac, but iOS mail keeps all the gunk off the computer, thankfully. I used the iPad as my camera before I got my iPhone5S, and I still appreciate viewing photos on the iPad better than anywhere else.
I have found Notes to be a great help as well. I am doing some construction in my house, and the iPad is good for quickly writing down measurements, materials needed, and being able to look prices up rightaway online.
Its form factor lends itself a little better for things like this than a laptop, and I find the phone a little too small for most of these things (let alone type on at length). Also, battery life on the iPad ensures you can work on it the entire day without worrying about charging, forget about that on the phone.
I said it is not a replacement for a laptop, but in fact I passed on my 2013 MBA to my son, feeling that the iPad could cover most of its duties, and I still have my MacPro for heavy-duty lifting. I do miss my MBA, it was the best computer I have ever owned, but I can't say that I'm handicapped without it.