Why should your new bathroom walls have plywood on them? Designing a home that can adapt over time without knowing how it will need to adapt is a difficult challenge. There are standards by which you can block for future grab bars that are designed to allow for someone in a wheelchair to use a bathroom. However, those that need to use a wheelchair late in life may not have the upper body strength to use those grab bars effectively. What if there is no wheelchair, but maybe just stability issues? Would you want to install the grab bars in different locations?
Using a plywood backer board, behind your waterproofing system allows for installing grab bars anywhere in the future. So, Why should your new bathroom walls have plywood on them? Because the future is uncertain. This strategy gives maximum flexibility to the bathroom as needs change over time. Making spaces as flexible as possible during construction allows your home to be a home for a lifetime. I know you don’t plan to need grab bars, neither do I, but it is good to plan for as many possibilities as you can while building your dream home.