My personal opinion is they are overpriced. Last time I looked even their website failed to spell out any advantages. If they cannot explain clear tangible benefits then, I agree with your question - why?
We were members of NSALG and are not anymore, that was a decision made before I got a plot. Asking why they left one comment was that insurance was available just cheap without them.
I do understand the need for pressure groups acting on behalf of allotments, and that costs money. I'd like to see their accounts to understand how the income is spent. That would help me understand the cost/benefits.