Because requiring an ID to register for eg. a facebook account is hard... how do you input your physical card into a computer? There's potential for abuse when taking a photo of it, and as soon as one photo of an ID appears online, everyone can use that.

By having people get digital IDs, you can require them to use it when registering for social network accounts, etc., because it's already in their phones and requires just one tap to 'sign' the account with your real identity. Want to complain about a local politician on r/uk on reddit? You'll need a valid UK account to even post there, and it'll be tied to your real name. Want to join a discord? You need UK ID and your real identity tied to it. Organizing a protest? Well, your real name is there, the police will be visiting soon.

Very good question, isn't it. The point of the policy is to make headlines. Implementation is a messy detail for the future.

Easier to store more information while preventing any non authorized to access them, like signing keys and biometrics ?