Another GP receptionist here. As others have said it's so you can be seen by the most appropriate person.
At the surgery I work at for example we have a first contact physio so if someone called wanting to see the GP because they have a sore back but there aren't any GP appointments available I can offer them one with the physio, who often has appointments sooner and can do any onward referrals if needed.
Our Advanced Nurse Practitioners can also deal with things like period problems, HRT or asthma and COPD and again, patients are often able to get seen by them sooner than they would with a GP. Or maybe you want to ask about the side effects of your medication: we have bookable phone calls with our pharmacy team the next day.
Our practice does a mix of telephone and in person appointments so sometimes we might offer a phone call with a doctor if you've said it's to discuss your recent blood test results, for example - here's a phone call next week with the doctor, or you can wait another week on top of that if you want to see them in person. You'll often find for things like that patients will be happy to take a telephone appointment.
If you're wanting to make a routine appointment I will ask for a very brief description, something like "gastro problems" , "skin issue" or "follow up" are enough for the doctor to have an idea, you don't need to go into huge detail.
There are certain things that the doctors tell us to watch out for when people are telling us what's wrong for example new chest pains or abdominal pains would warrant a call from the duty doctor. Sometimes we might check with the doctor and they advise us to tell the patient to dial 999.