Forgot to mention and now it's too late to edit: the most common way to do quick tests during development was to only build a few DLLs or .sys files and replace them on a running system. Then your team would have a set of branches that build every night.
But that's just Windows. What you can get a Google is a full test run over _everything_ that your change affects. This lets you ensure that an obscure change in behavior will not break others, including products built using your library as a remote, transitive dependency. You also get to fix bugs globally. Say you had a really shitty internal API that was causing problems or slowing things down. You get to actually go in there and change that API, and update callsites in one atomic commit. You can also make sure that you're not breaking anyone's build or tests by introducing your change. There are teams at Google whose purpose in life is repo-wide "code cultivation". Finding issues and fixing them globally basically. This just doesn't happen at MS.
