Designing with accessibility in mind makes software better for everyone, period.
You design a clear, visible, distinctive scroll bar and buttons? Boom, you've catered to everyone with bad vision (for any reason: from blindness to eye surgery), reduced motor skills (for any reason: from arthritis to old age to hand injuries), non-technical users (so they don't have to hunt around the interface hoping to discover hidden features).
And this goes for everything in software.
Disability is a spectrum, and you yourself will be disabled in one way or another, multiple times, during the course of your life. Be it from old age, surgeries, injuries, strain from sport or mundane tasks like holding a baby.