Any interview is designed to get that persons side of the event and to ask them about the allegation made against them.
Person A assaults person B. Person A gets arrested and Person B provides a statement with some CCTV.
Person A is arrested as there are sufficient grounds in this example (on going incident and the victim is at risk). Person A is interviewed to get their account (did you do it or not). if you didn't do it, how do you explain these injuries or explain this CCTV footage. Person A can elect to answer, or go 'no reply'
Because of the wording of the caution (it will harm your defence.......) - if they are asked similar questions at court but given a different answer - the court can draw an inference from it - such as if they go no reply, when they go to court, the court can draw an inference as to why they didn't answer that question when they got interviewed by police.
A voluntary interview is just that - there is no necessity to arrest them so they can be vol interviewed. They are not under arrest and they are told they are free to leave at any point. So, they can up and walk out at the start.
They can't then be arrested - as it will not be a vol interview. Besides which, they have been told they are free to leave at any point. If they do this, they are effectively waving their right to be interviewed. The case file will just proceed as normal to the court without the interview of person A
In essence for a vol interview - I can't say:
"This is a voluntary interview and you're free to leave at any point - but if you do try an leave I will arrest you"
As it won't be a voluntary interview
If you simply refuse to attend for a vol interview, it can be used as ground for arrest. This would depend on the offence, the reason why they are avoiding police, are they on the run, the custody sergeant etc. If they don't turn up for an interview, what is the prospect that they won't turn up at court?