It's not strictly needed for a good finish, but will make it alot easier if the brick wall is uneven or slightly out. Generally this is a trade off for cost vs ease of installation, for just a WC the benefit becomes more skewed, but if you are striving for the very best, the board will allow you to create a perfectly level and even surface before tiling.
Aslong as the walls are good then spot bond is not an issue.
Thickness will depend on your situation.
If the bond is questionable then mechanical anchors are placed aswell.