Why do scientists use the Latin names when describing a species?

To avoid confusion between languages, and within the same language, about exactly what species you are describing. 

The fish in the photo above is Amphiprion ocellaris.  

For identification I could go to my favorite identification book for fish in Okinawa: Reef Fish Identification – Tropical Pacific

In this book Amphiprion ocellaris is called the False clown anemonefish.

If we check on Wikipedia for Amphiprion ocellaris we find a wide range of common names.

“The ocellaris clownfish, also known as the false percula clownfish or common clownfish, is a marine fish belonging to the family Pomacentridae, which includes clownfishes and damselfishes. ” Wikipedia

However, my preferred source for fish names is Fishbase which gives the common name as clown anemonefish.  

Then again, for millions of children around the world,  Amphiprion ocellaris is simply known as Nemo.