Because it is entirely plausible that you need to add a folder where only you know the ID that another device has already chosen for it. Often you will get a sharing “invitation”, but otherwise you will need to enter the ID manually. Conceptionally, there is no difference between adding a “new” shared folder (with a new, unique ID) and “joining” a shared folder which has already got an ID from somewhere.
If I remember correctly from the ancient Syncthing history, originally the ID was the label, and what is currently the label was only introduced later.
As mentioned, that feature didn’t always exist. But you might also want to configure a device before it’s connected to the outside world, if you’re in a cabin in the woods or going to ship it to a research station on Antarctica.
Or if you are in France like me and there is a probability of snow tomorrow (I estimate about 1 mm (0.04 in)) and the whole Paris region is preparing to sustain this.
To be able to enter the folder ID is a necessary feature of you want to connect two folders (perhaps very far from each other) where you have done an initial sync by other means (such as copying files from a USB stick):