Young children, due to the stage of their physical development, draw from the shoulder rather than the elbow and wrist using large arm movements. At this stage they often prefer vertical drawing and painting surfaces as it allows a free range of movements. This is often why young children will write on walls, not because they are being naughty but because it just feels comfortable and so more enjoyable.
Drawing and writing on a vertical surface is important at this stage as it helps young children develop the wrist strength and flexibility they will need later on to hold a pencil correctly for handwriting.
The jump from a vertical to a horizontal writing surface can seem too great for some children; due to their stage of development. These children may benefit from the paper being positioned on a sloped board.
If you are not sure whether a child needs a sloped board for handwriting, instead of buying a specialist board, you could make one. Try using a ring binder or lever arch file stuffed with magazines and newspaper to make a sloped board. Tape the edges to stop the papers falling out; you could cover it in sticky back plastic to give a smoother finish to the board. The advantage of this is that you can make them to any angle of slope. Try a few to see which, if any, the child prefers.
A homemade sloped board is just as effective as a bought one. Often a child only requires one for a short amount of time and quickly moves to writing on a horizontal surface. For a few children a sloped surface may be required for a few years, or indefinitely, in which case a purpose bought sloped writing board is a sounder investment.