Anything frequently used is probably best kept on your device.
It is, thanks to a MicroSD slot allowing me to have lots of storage. These days, I've got about 57Gbytes with me at all times.
If data is that expensive for your phone then why not have a broadband connection?
I do, at home, but not when I spend weekends up north, where the closest building is a gasbar/convenience store about 2 hours' drive away.
Out of curiosity, what do these billions of people do with smartphones that don't have internet connectivity?
That is NOT what I wrote. I wrote that "billions of people subjected to flaky data reception and/or extremely limited data plans". That you cannot fathom this very real situation just shows how lucky you are. Believing everyone else is as privileged as you seem to be (connectivity-wise), is short-sighted, and denotes that you are not much of a traveler. That, in itself is no sin, but your skepticism about data connectivity being problematic in certain ways for many people is quite puzzling.
Pretty much all I can do with mine without Internet is make calls, text and take picture
Well, perhaps it's your case.
For me, the smart phone is much more than just a streaming device/browser/phone. A smart phone loaded with a bunch of apps, is nothing less than an ultra-portable computer. A computer does not necessarily need to be connected to the web to be useful. If it has ample storage, the Internet becomes much less important, and the same holds true for a phone.
My phone gets a lot of bandwidth-less use. It's the ultimate deep woods cabin and camp site jukebox and video playback device. It's no tablet, but still, I read books on it. It's also what I use to keep and edit my Journal. My work hours are logged on a spreadsheet which I edit and save on the phone itself. (I don't have a regular office job). I use it to have a read out of my GPS coordinates and use it's compass. I use it to ID vegetation and birds with my botanical and birdwatchers' guides. I also have a folder with a whole bunch of recipes saved from the web, in many deferent formats, including some in video form, so I also use it for cooking. The timer on the phone is also quite sweet for cooking times. It's Notepad is a regular life saver. I also keep a few catalogs of all owned hard copy media, which avoids double purchasing of CD /DVD/ Blu-Ray. I also keep a locally stored "wanted list"; because my Amazon list can be hard to connect to Inside a big shopping center, and HMVs often have better deals than Amazon on the type of titles I like.
From April to October, I spend my weekends camping in out-of-coverage areas. I live in a city of a 3 and a half million people on the east coast. Yet the first "No reception at all" zone I encounter is just 3 hours' drive away. On the 11 hour drive to the city where my parent met, and where 40 thousand people live, there are 3 "data blind" spots and one 2 hour "No reception at all" zone.