Accessibility refers to the openness of your resources to be reached by all students.  All students need equal access to class materials regardless of any accommodations, but accessibility goes further.  Accommodations refers to adjusting your material on a case by case basis when the need arises, accessibility refers to making your material easily available to all students all the time and has added benefits beyond meeting accommodation needs.

For students who do require accommodations, having a class already set up to be fully accessible is more welcoming and puts less stress on the student.  For students who don't require any accommodations, having  class already set up to be fully accessible can be much easier to access resources on personal devices, have easier time comprehending material, and able to access everything in a method that works best for each individual student.