Searching metadata only is often very useful because it ensures that the words you search for are actually important to the work, and not just mentioned in passing. But metadata searching goes well beyond that in its power.
Most metadata is structured, which makes it more than just a bunch of words. Each piece of structured metadata is associated with a field that tells you how it relates to the object. This enables powerful searching options. For example, you can search for "shakespeare" and specify whether you want something called Shakespeare (title), by Shakespeare (author), or about Shakespeare (subject). That's not something you can do in Google!
In addition to generic fields like title, author, and subject, metadata can also include specialized fields useful for certain disciplines. For example, most metadata systems have a date field for date of publication, but metadata schemes for history books and articles often have a separate field for the dates the work is about.
Finally, good metadata will use controlled vocabularies. This is especially useful for subjects, where many variant terms may be in use by writers. The classic example is film, aka movies, motion pictures, or cinema. A controlled vocabulary picks one standard term for each topic, so no matter what words the original author uses, or even what language they write in, the metadata will be consistent.