Making decisions to purchase products like iPads is straightforward when buying wifi only. The model size and color is the only other choice. Adding cellular at least in Australia however does need extra thought. The type of customers I sell iPads cellular models are business people ie outside contractors, real-estate personal, managers and company owners truck drivers and sales people to name a few. Another group are Australian mature adults or grey nomads whom are traveling around the world. The latter buy mainly cellular minis for portability and having gps is an added benefit. They can also interchange sims in different countries without much effort. in addition they typically have low cost phones like Nokia that are used as phones for calling and text and are not interested in smart phones
Using a phone to tether is a good choice for some regardless if it is Apple, Android or Windows smartphone. Be aware one may need to charge the mobile more frequently as it can drain the battery. Also some windows smart phones do not tether easily so check it out first before buying your iPad. The other item is the added cost of the increase in data usage on a tethered phone so make sure it is not more than $15 a month. The fact is in Australia at least prepay plans for 12 months with 12gb limit costs $180 year which is only $15 a month really not a lot of extra money for the convenience of having it built in. Some users are also locked into phone companies that have poor service and in some cases donot allow tethering.
The third option is mobile portable wifi rechargeable hot spot modem.
These come in 2 flavors 3G or 3G and 4g both support up to 5 devices and come with prepaid 3gb data for 30 days included. Cost of these modems range from $50 to $150 depending on brand and models They suit families whom often have several iPads and or any mobile wifi device like e-readers and laptops and are great for traveling as well as in the home. These are also popular with users whom are renting where they do not have access to broadband as they support other devices like printers etc as well. This type of mobile device is available in most countries so check it out.
So there are reasons why users buy cellular instead of going with the above options and that is why Apple sells so many.
The long and short of it is if you are buying an iPad make a list of your needs and if you really need to access the net other than using wifi consider your smart phone or use a portable wifi hot spot modem and finally a iPad with cellular also look at the network monthly costs prepaid or contracts and purchase the iPad which suits you and your needs.
Hope that answers some questions