I snapped a ligament in my knee and my physiotherapist asked me to have a go on one of these.
I think the idea was that it would improve proprioreception (i.e. our ability to balance and know what our body is doing when our eyes are closed) and this in turn would reduce the risk of reinjuring the knee.
I did consider my balance to be okay, e.g. I can walk along scaffold poles without holding on to something, but still I found balancing on a wobble board to be quite a challenge. As such I still feel motivated to use it, months later.
If you have zero balance you can use it whilst holding on to something. I would have given 5 stars but £27 for a bit of wood seems a bit steep. It probably helps skateboarding/surfing ability but I am not so cool as to be able to confirm that.