but surely people who do need these benefits shouldn't then be able to save
It's aiming to help people budget for the medium term and not spend every penny they have every week/month. If people on a low income start to save for Christmas 2019 now, they will have some money saved up, and won't need to borrow. Same for days out in the summer holidays or if the washing machine breaks.
I think it's a really good scheme because you're not committed to saving every month and the bonus is based on your highest balance, so there's no detriment to taking money out if you need to. MIght be hard to get going and get into the mentality, but will make a big difference to those who do manage to put a bit by and get a bonus.