Why use a Smartphone or Tablet for outdoors activities?
Let me start by saying "I practise what I preach"! I have owned two GPS units but now my only aid is a smartphone. It cost £110 without a contract and I have used it for hiking in Britain, Spain, Oman and many other places
When I am hiking, I use a 'moving map' on the smartphone that shows me exactly where I am as well as recording a track showing where I have been.
If you buy a dedicated GPS unit then yes, you will buy something more rugged and with a longer battery life, but you are usually tied to the maps that the manufacturer sells you. If not impossible, installing your own maps is tricky or very restrictive. The GPS unit has its own operating system and may offer a few games but nothing else. Maps can be expensive : think £100 for Ordnance Survey Landrranger for all of Britain or £60 for one National Park at 1:25,000
A smartphone gives you a wide choice of different Apps for navigation. Almost all of these give you access to a range of on-line maps - Open Street Map is usually available and many others, some of which you pay for. All the applications I know of allow you to add your own maps. Free, legal, high quality maps of Spain, US, Canada, Norway and many other countries are available and can be loaded with the help of software that is either free or costs, maybe, £10. Other maps are available such as the excellent Tour and Trail maps of the Canaries, Balearics, Madeira and parts of Spain.
In addition to the navigation apps, there is the whole Google Play or iTunes library to choose from and the smartphone usually incorporates a camera, too. GPS signals are free and, if you put maps on the unit before you leave home, there are no data roaming charges to pay. And when you trade-up your smartphone, the apps and maps can be carried across for free.
The two big disadvantages of a smartphone are easily addressed. A waterproof Aquapac or Otterbox case protects it. A rechargeable USB battery costs £20 to £30 and will run a smartphone for days (if you are not making or recieving calls put it into Airplane mode as well).
[A word of caution about tablets. Whereas almost all smartphones have GPS built in, there are many tablets that do not. Without the GPS (or access to a GPS device with Bluetooth) the apps will not work.]