While that's true, certifications aren't designed to teach you everything you need to know about everything, ever; they do, however, try to guarantee a minimum level of knowledge. Studying for certifications through the course material will teach you a lot about whatever it is you're trying to learn. Whether or not you take the test at the end is purely your choice, but HR departments definitely look for them, especially when you don't have years of experience through the industry.
I can also vouch for the Cisco certifications; CCNA/CCNP will teach you a lot about how networking. This is all certainly info that you can learn on your own, but it's nice to have a prepared course and curriculum.