What does a high gain 3G or LTE antenna do?

A good high gain mobile antenna will improve the signal to noise ratio, which will help you to get better data rates than was previously possible.

Keep in mind that your signal strength is also influenced by other factors, such as the walls and other buildings between you and the signal source, the length and quality of the cabling to the antenna and even things such as the number of users that your service provider has allowed to share a certain bandwidth with you.