I was wondering if anyone could explain to me a bit more about the sheeted marbles, and what, if any, benefit they bring to a breeding programme.

In marbles there is the black pattern and the background colour.
Sheeted marbles are marbles with a high ratio of black pattern to background colour and hence can look quite quite black all over, especially as kittens.

Sheeted marbles are usually the product of a rosetted program. The rosetted cats also have a high ratio of pattern to background colour so when a marble is born the ratio is translated into the sheeted marble look.

I do not believe they add anything in particular apart from the marble gene. Their genetics are dictated by their parents as regards rosetting, so coming from rosetted lines means they are highly likely to give rosetted kittens if mated to the right mate.
They are often cheaper than their highly rosetted more desirable litter mates and have the potential to produce as good kittens, so may be a way of procuring some desirable lines from highly popular catteries, but the downside is they will add the marble gene to your program. All offspring will be marble carriers and will have the tendency also to produce sheeted marbles with a marble carrier mate, kittens which may be difficult to sell.
The desirable open marbles of marble to marble breeding will be elusive usually.

Thanks Elaine