A sequence is not four independent emails. It is one conversation with delays in it, and the follow-ups only make sense if you can read them next to the message they follow. Reviewing them one form at a time hides exactly the problems that matter — a follow-up that repeats the opener, a step-three reference to something you cut from step one.
The rebuilt sequence editor puts every step on one page: subject, body, delay, in order. The multi-step preview renders the whole sequence against a real lead from your segment, with variables substituted, so you read what the recipient reads.
If a template variable is missing for that lead, you see it in the preview instead of discovering it when message generation fails.