Enroute alternate is only an issue in non-ETOPS flights for fuel reduction requirements, as a previous poster responded. It enables the operator to plan a flight with a reduced fuel requirement, using 3% contingency fuel rather then 5%, by taking into account the capability to land before destination in case of an adverse fuel use scenario.
Example flight from Canaries to UK, B738 trip 13,000kg thus contingency fuel requirement of 5% would be 650kg, 3 % would be only 390kg.
If this island would be takeoff performance limited, such as ACE, this would enable the crew to take on some additional bags or passengers. Thus by designating e.g. OPO as an ERA it would enhance payload if required.
Think of a BBJ/B737 ETOPS flight flying AMS-IAH, trip ~29,000kg thus contingency fuel of 5% would be 1,450kg vs 3% 870kg this becomes even more pronounced. Such flights may even benefit from decision point procedures enabling further fuel reductions thus extending range/payload...