Search engine crawlers are ultimately HTTP clients too, so status codes directly affect how a page gets indexed and reflected in rankings. Normal pages should return 200, and only pages that return 200 become proper candidates for indexing.
Redirects matter especially. 301 Moved Permanently transfers the old URL's link equity to the new URL, whereas 302 Found is treated as a temporary move and does not pass on ranking. For permanent changes such as a domain migration or an HTTPS switch, you should use 301 (or 308, which preserves the method).
For deleted pages, return 404 Not Found or the stronger signal 410 Gone. Conversely, a 'soft 404' — where a nonexistent page returns 200 — wastes crawl budget and should be avoided. If server errors (5xx) persist, crawlers reduce how often they visit, so stable responses and appropriate status codes are the foundation of SEO.