Arial should have never come to be. The only reason it exists is because changing it a little and giving it a new name meant Monotype didn’t have to pay Linotype license fees.
Arial was originally it's own font (Monotype Grotesque), but got squished to match Helvetica's metrics which is a good example of how fonts can be different and similar to each other.
As you can see, all the areas in which Monotype Arial is different from Linotype Helvetica are borrowed from Monotype Grot. The reason to make such a monster is purely financial.