Please contact me, either by phone or by e-mail, if you have any questions. 和 Please contact me either by phone or by e-mail if you have any questions. 的差別在哪裡？如果不好說明，請提供一些例句。

When two comma are like that, they are just adding extra information that is not needed for the sentence to make sense.
Example: "This is my dog, who is normally not as jumpy, named Jake." It still makes sense to say "This is my dog named Jake" and the in between stuff is extra information.