Looking at it from your creditors point of view, they won’t be receiving all their money back in the IVA, but you have a lump sum in your property which will benefit at you some point.
This is why they ask you to try to release it. They can’t take all the equity up to a maximum of 85%. In bankruptcy you would probably lose the property entirely so it may be that releasing equity is a favourable option. If you manage to gain a reasonable offer at anytime during the IVA you can remortgage there and then, however if you are struggling, the Insolvency Practitioner can help you get the offers.