Preview deploys as a real code review tool, not just a demo link

Vercel (and similar platforms) generating a live, deployed URL for every pull request usually gets used for one thing: sending the link to a designer or product owner for visual sign-off. That's a real use, and it undersells what the same URL is good for during actual code review.

Diffs don't show you what changed visually

Reading a diff tells you what lines of CSS or JSX changed. It doesn't tell you what that change actually looks like rendered — a spacing tweak that reads as reasonable in the diff can look clearly wrong once you see the actual page, and a diff that looks alarming (a large refactor of a component's internals) can turn out to render identically to before. Opening the preview URL alongside the diff catches an entire category of regression — visual breakage, layout shifts, a component that renders but looks subtly wrong — that reading code alone reliably misses, because code correctness and visual correctness are different questions.

Testing on a real device against a real URL

A preview deploy is a real, publicly reachable URL, which means testing a responsive layout change on an actual phone is just opening the link on the phone — no local tunnel, no port forwarding, no "works on my machine at this specific viewport width" uncertainty. This matters more than it sounds like it should: a layout bug that only shows up on an actual iOS Safari viewport, with actual touch scrolling, is one that a desktop browser's responsive mode can miss entirely, and a preview URL removes all the friction that normally makes testing on a real device inconvenient enough to skip.

Where this fits in review

The practical shift: treat the preview URL as part of the review checklist, not an optional extra — reviewing a UI change without opening its preview deploy is closer to reviewing it half-blind, especially for anything touching layout, responsive behavior, or interaction states like hover and focus that are genuinely hard to evaluate by reading JSX alone. It's already there, generated automatically, for free, on every PR — the only missing step is actually treating it as a required part of review instead of a nice-to-have link.