Because that’s the standard and your paper will get rejected if you don’t follow the standards
The citation should serve as a unique identifier for the source material you used. As a trivial example, let’s say that your reader wanted to look at the material from page 103 of the Smith work that you cited. What happens if the pagination is different between the 1964 edition and the 1979 edition? This will tell your readers which one to look at.
It’s also important to let your readers know when the material was originally published. Things have changed a lot in the sciences (both hard and soft) over the years, and if your readers are unfamiliar with the cited author the original publication date can provide useful information.