iPhones and iPads
We can unlock iPhone and iPads (and any other phone too!) from most network across the world. Allowing you to choose what network and what tariff you want. This can save you money.
If you unlock your phone you can keep your existing phone rather than changing it when you change contracts. Phone companies are very good at selling you a new handset you don't need to entice you to spend more with them. A sim only contract is far cheaper than one with a handset ? Do you really need a new phone or more data ?
Higher Selling Price
If you are looking to sell your phone it will achieve a higher selling price, unlocked, as it will be more attractive to people who have sim only contracts, and easier to advertise an unlocked phone as anyone can buy it not just people on say 'Vodaphone'. It'll see faster.
You may be on a particular network and moved house and found another network has a better signal according to locals and rather than signing up to a new phone and a new network you can unlock your phone and put a sim only deal in. Far cheaper and you won't have to sign up either if you want to use a PAYG sim.
If you travel Abroad
If you travel abroad you may want to put a local sim in your phone in that country as it maybe a cheaper way of calling, using data. An unlocked phone will allow you to do that. Especially if you have an iPad using data that doesn't allow to use much of it roaming on other networks.