Even for internal apps, it might pay to think about accessibility... e.g. imagine someone who got hired to use the "internal app" was a Marine vet who lost vision to an IED or something... if your company is following the law, you have to provide the same opportunity for him to work there and use that "internal" app as everyone else. Maybe there's some reason why that would never be the case, but devs tend to think of the issue of accessibility much too narrowly.