The main difference between the 2 is speed. For our example, we'll think of the UART as the counter person at a hamburger stand. Wait a minute. Not all data is compressible. Most graphics don't compress well.
Text compresses well.
But only a tiny amount of all the information on the internet is text. However, this is necessary due to the way analog telephone equipment works. So, a 33.6kbps modem can still go faster than a 28.8kbps modem because neither one ever reaches it's full potential. They are usually working with data that is already compressed, or graphics, neither of which can achieve 4 to 1 compression. See for yourself.