Cookies are how GA4 knows it's the same person coming back.
We validate cookie domain settings (so subdomains share the same visitor identity), check cookie flags (SameSite, Secure, HttpOnly), inspect expiration dates (GA4 default is 2 years), and identify potential tracking cookie issues like wrong domain scope or browser blocking.
Cookies are how GA4 distinguishes a brand-new visitor from a returning one. If cookies are set on the wrong domain, expire too quickly, or get blocked by browsers, returning users get re-counted as new every time — and your customer journey, retention, and loyalty metrics fall apart.
A subscription box company has cookies scoped only to www.brand.com but their checkout is at brand.com (no www). Every visitor who hits checkout looks like a new user in GA4 — making it impossible to measure the conversion rate of repeat browsers vs. first-timers.
A hotel chain's marketing team thinks their email campaigns aren't working because click-throughs from email show as 'new users' instead of returning loyalty members. Turns out cookies are set to expire after 30 minutes instead of the default 2 years.
A SaaS site has the SameSite flag set incorrectly so Safari (with ITP) blocks the GA cookie entirely. They wonder why their Mac users have 'one-and-done' sessions and underinvest in product features Mac users actually love.