@RoyalCorgi
The whole point of it being a voluntary interview is to try and get the suspect to let their guard down and for the police to get evidence against them. When you are arrested and interviewed it's human nature to be afraid, want a lawyer, and clam up. This happy shiney rainbow voluntary interview is meant to put the suspect more at ease. The police will use phrases like "an opportunity to tell us your side of the story", be reassuring, act nice, "you can stop at any time". The whole point is they are collecting evidence against the suspect. A person might not insist on legal representation during a happy clappy voluntary interview. This is a huge mistake.
Lets say the police already have some evidence against a person, but it's borderline, 50/50 at best, they know it's weak. So they invite them in for a voluntary interview. The suspect thinks they are just clearing the whole misunderstanding up, giving their side of the story, they don't bring legal representation. But during the vol interview they say something that the police consider good evidence. It's no longer 50/50, they have all they need. The person is arrested, charged, and it's game over, next stop court.
Without that vol interview the police may not have done the arrest for an interview. If the police did arrest the person may have had a lawyer present who stopped them saying anything incriminating, now the police still have very little and have to drop the charges.