These are very useful if you use thin paper or press hard when you write. You put them a page or a few pages beneath the writing paper and they provide a nice surface upon which to write. They prevent your pen or pencil from leaving pressure marks on the pages beneath and really make writing look nice. They are also useful to put under your hand as you reach the bottom of the page of a book. This prevents your hand slipping awkwardly off the edge of the writing area.
Looking at this picture, on top left is a Hobonichi A6 pencil board, a Nanami Paper soft green A5 writing mat and a Nanami Paper A5 hard clear writing mat. On the bottom are three thinner shitajikis from eBay. All of these are available in different sizes and can be cut down to size. The bottom three are very thin and light can be punched to form a dashboard or writing board for a Filofax without taking up too much space. I have not bought from this seller but he or she sells clear writing boards in all colours. The A6 writing board would work for paper larger than A6 as you just move it down as you write lower on the page.