I don't have a place board but when I've been on training courses they've been used for getting the dogs to recognise a specific place to start work on or from.
It was also used to teach our dogs to place both paws on the board starting with C&T-ing a touch with a nose building up to a paw near, then on, then both paws on (I now use an upturned plant pot for this).
I also used it as a waiting place whilst throwing treats left, right and behind; dog sits and waits until directed with arm movements which direction to go in to seek out the treats. I don't use a board for this now but it helped initially to have Plum sit and face me on an object rather than just the grass.
At the moment I've been trying to teach Plum to sit and hold a dummy in her mouth. Whilst I don't have a place board I have a paving slab in the garden which I use instead when I want to practise certain things. Interestingly, when I direct her to the slab by pointing and saying "place" she automatically sits (because that's what "place" has meant) and she will take the dummy in her mouth whilst sitting, whereas on the grass, she would sit, but as soon as I gave her the dummy she would stand. So "place" helps her recognise something new is happening or something old is being practised.
Profuse apologies for my longwindedness!!