The commercial case for accessibility is usually made on audience size: a meaningful share of your visitors have a disability, and an inaccessible site excludes them. That is true, and it should be enough. But it undersells what actually happens when a team does the work.
Accessibility is a forcing function for clarity
To give every form field a real label, you have to decide what the field is actually asking. To write alt text, you have to know why the image is there. To fix heading order, you have to agree what the page is about. Every one of those is a content problem wearing a technical costume, and fixing it helps everybody.
The pattern repeats. Focus states make it obvious where you are in a long form. Contrast minimums kill the grey-on-grey microcopy nobody could read anyway. Target sizes stop mis-taps on mobile. None of these are niche accommodations.
Where the real work is
Automated tools catch roughly half of the issues in the standard, and that half is the easy half. The rest is keyboard walkthroughs, screen-reader passes and judgement about whether a control makes sense out of context. Budget for it as real work, because it is.