What Eric and Baelari said it both true!

And also, think about this.
You live in England and you have a friend who want's to send you a "Blacksnake". You think it's the harmless Black racer from USA, but instead you recive a deadly Blacksnake from Australia! Not so fancy!

Common names are extremly good to create uncertainty and confusion when talking to people across the world. With the scientific names, you always know for sure. If you are only talking to people within you area, then sure, it might be somewhat less needed.
On top of this, some species not does have a common name in every language.