I have a phone. It's fine.
I do not want to rent a phone.
I do not want to have a phone tied to one network.
I will happily go out and buy a phone (even on finance) just to be able to change networks at the drop of a hat.
Consequently, a SIM-only deal has to exist for me. And given how much cheaper people like giffgaff can be than the major-players, I'm saving money.
I just bought a 4G Wifi router (don't ask about landline broadband... it's basically non-existent despite being in a major city). I bought an Huawei one that *I* like the look of and looks and works how I want it to. On any of the major carriers that sell the same box, they want to rent it to me for about £10-15 a month on a 24 month contract. The box cost £60. Why on earth would I do so?
Instead I bought it outright (hell, I could spread the payment via PayPal Credit if I really wanted to, and it would still be cheaper). Then I picked up a SIM-only deal. Because it's SIM-only, the "deal" is the same data package. But it's cheaper. Though, because I want only a monthly contract, the numbers are fiddled to make it a tiny bit more than it should be but still a lot cheaper than the very same package with a router/phone included.
So... why would I buy via them?
It's about reducing unnecessary middle-men. No provider I've ever used has had such good service that I want to throw money at them just because. None. I don't mind "spending more to get good service". But I'm not going to give them a 2-year credit agreement on a little box I could just buy (and get an unlocked version) just for the sake of it. I'd actually rather buy the box from anyone, then buy the SIM from the company I like, and then knock it up a notch in terms of data/calls if that's what I wanted to do.
Telecoms carriers "leasing" their phone to me died with the dissolution of the BT-phones-only network, as-was. I see no reason to jump back 40 years in that regard just because I carry my phone with me rather than tie it to the wall at home.
And certainly not when the premium attached (24 month contract, vastly more than the device cost, huge cancellation charges, etc.) is sometimes 5 or more times the cost of the device.
Also, when they annoy me - which every company manages to do at some point - I just go with someone else next month and take my number with me. I will happily pay slightly more for that kind of freedom of choice of provider / device without the telecoms provider getting to have a say in it.