The dashes make the dates easier for users to read because they make the date into one word rather than three separate words that the user must combine into one word in their heads.
You should strongly consider using the ISO date format: “YYYY-MM-DD” including the dashes.
There are 3 good reasons for this:
1) when alphabetized, this format sorts all of the days of a particular month into that month, and all the months of a particular year into that year, which is what you want almost all of the time
2) it is unambiguous — some regions use MM-DD-YYYY and some regions use DD-MM-YYYY so is 11-09-2015 November 9th or September 11th? there is no way to know, whereas with 2015-11-09 that can only be November 9th
3) it’s the ISO date format and you get all the benefits of standardization such as it is understandable worldwide by humans and it is easy to work with in software development environments — for example if you want to localize your dates for a particular region, there is likely a built-in function in your programming environment that creates a date object from a text string containing an ISO date, then you use that date object to write out any other date format, but you still store your dates internally as unambiguous ISO dates