SIM Only contracts have a number of advantages. For one thing, they’re cheaper than getting a contract that includes a new phone. They’re also almost always shorter term, generally coming in at either 30 day rolling contracts or 12 months, so you have more freedom and flexibility to change your tariff.
SIM Only contracts are also ideal if you’re happy to keep your current phone – or want to buy a new one outright rather than paying for it monthly.