Why are accent marks important in french?

It's important to use them because they are part of the spelling of a Word. In general, accent marks tell you how to pronounce a Word just like individual letters do (for example, é sounds like "ay" while è sounds like "eh"). Of course a French person will still easily read something written without accent marks (which is done sometimes when a message is in all capital letters) just like we could read something in English without any apostrophes and with lots of spelling mistakes. But that doesn't make it right! Always write and type with accent marks!