Hearing aids can do both. However, everyone's hearing loss is different, not all hearing aids perform equally, and not everyone will receive the same benefit. No hearing aid will restore normal hearing, but they do help millions of people around the world to hear better. Today, hearing aids are available that are much more than mere amplifiers. They can provide selective amplification across frequencies - thereby providing differing amounts of amplification as needed for a particular hearing loss.
In addition, they can reduce certain types of background noises to make listening comfortable, provide a preference for sounds from the front as compared to sounds coming from behind the wearer, automatically cushion very loud sounds, and even store information about the wearer's listening environment. And that's just a few of the features available. Some people have poor word recognition ability, meaning that due to inner ear damage, they don't understand words clearly, even when things are loud enough. Hearing aids can't fix a damaged auditory system. Speak with a professional to understand the types of benefits you may be able to reasonably expect from your hearing aids.