Initially, I didn't see much need for a dictionary and was perfectly happy without one on my BeBook. When I got the PocketBook 360 it came with a dictionary function and there were some free downloadable dictionaries available so I loaded them.
I find the ease of being able to look up unusual or archaic/dated words very handy, and have made extensive use of the dictionary function even for modern writing when the author uses some intriguing wording.
As has been mentioned previously, I wouldn't consider it essential, but it is a very nice-to-have feature on any eBook reader.
