Before CI existed as a practice, integrating everyone's changes was a periodic, manual, and often painful event — "integration day," where days or weeks of parallel work collided all at once and the resulting conflicts and breakages were often severe enough to blow through the ostensible fix time. The core insight behind CI is to make integration boring by making it constant: every push is tested against every other recent push immediately, so conflicts and regressions are small, fresh, and cheap to fix, instead of large, stale, and expensive.
The same logic extends past just "the code compiles": a pipeline exists to enforce, automatically, on every single change, whatever standard the team has decided the codebase must meet before it ships — tests passing, code coverage above a threshold, linting rules, security scans, whatever's been chosen — without needing a human to remember to check for it, and without a change getting through because someone forgot, was in a hurry, or didn't know the rule existed. A pipeline is a policy made mechanical.