I always assumed it was for familiarity + perhaps to further enable people to use the tablet without a keyboard. When you first try out a drawing tablet, it seems like it makes sense to have it, but I've personally never used it. It's so tedious
Wacom even has an option to set one of the stylus buttons as the eraser. I've found that to lag a bit, so I usually just e and b. By lag I mean that sometimes the last stroke ends up being a normal brush stroke. The eraser tip does work just fine
Maybe the biggest benefit of using the eraser tip would be how Photoshop handles it. Photoshop retains the same brush when you are using brush tool and switch to the eraser side of the stylus. If you toggle the tools with
b, you may have different style brushes in each tool.
I do like to keep my eraser tool separated from the brush tool. If I'm using some sort of artistic scatter brush and then if I want to erase, I most likely will not want to use the scatter brush to do it.
Not all applications do it the same way as PS. Some applications may just kind of switch to the eraser tool when you are using the eraser tip ( This could be outdated information. )