Actually, a visual once-over can stop you from wasting time on wild goose chases. @the_ivan has a point about codes being fast, but they're not always right. Error guides assume everything is connected right, which isn't always true. Spotting a blown fuse or chewed wire takes seconds and rules out big repairs. Skipping that step means you might replace a good part based on a bad code. Trusting your eyes first just makes sense.
Error codes don't always tell the full story and can send you on a wild goose chase. I've fixed problems where the code said hardware failure but it was just a loose cable. Starting with a quick visual once-over has saved me hours before.