[steven-xu]

In the United States, the common interpretation of Section 508 requires that vendors and vendors of vendors of federal government contractors adhere to WCAG 2.0. Building it from the start prevents you from having to redesign parts of your work and churn your users when you choose to pursue this market.

A minority of your users may have permanent disabilities but many have many have temporary or situational (looking at their phone in a bright coffee shop, forgot their glasses) disabilities, and building to common a11y standards allows you to use off-the-shelf tools (e.g. axe) to easily and semi-automatically help maintain that you can deliver a good experience in these circumstances.

Some surveys I've been aware of find a surprising amount (1/3 to 1/2) of users use screen magnifiers. This isn't common among developers who have 30" IPS monitors and good or well-corrected vision, but it is more common in the general population. Anecdotally, many people have awful displays too.