OptBlue has changed the way merchants do business when it comes to accepting AMEX. Previously all AMEX charges had to run on a separate account directly through American Express. This meant that merchants had to deal with reconciling two sets of processing statements as well as dealing with extra fees from American Express.
With OptBlue your AMEX transactions run through the same merchant account that handles the rest of your business. That way all your charges are available on one processing statement. That means less work, and a simplified back-end process.
As long as you are processing less than $1 million in AMEX sales per year you are eligible for OptBlue. If you go above that threshold, you will have to go with one of AMEX established credit card programs.