Why do computer need a user interface?

Without a user interface of some sort, a user cannot interact with the computer. If you cannot interact with a computer, you cannot tell it to do something, and cannot retrieve information from it.

Answer

Back in the day, there was no "GUI," a graphic user interface. You had to have a lot more computer knowledge to get the information you needed--most requests were in a programming language, like Basic. With interfaces, you could quickly access functions minus the programming issues.