cubic metres not linear
Linear metres is meaningless and does not have a cubic metre equivalent.
Cubic metres enable you to give a full three dimensional size to your collections whereas linear metres just gives an idea of the length of shelves your boxes sit.
If you are subject to Cipfa you measure in cubic metres anyway. It's not difficult to work it out so there should be no reason for anyone considering accreditation not being able to provide the size of their collection in this way.
Why the contribution is important
Cubic metres enables more accurate idea of how big collections are and is already the standard measure for many archives.