Umm...The Arc has a guild component to it...the goods it produces, deposited automatically in the treasury. As for why a guild wants medals, medals are what you pay to siege GvG sectors in the all ages map.
Oh, indeed, both Voyager and The Arc gave guild goods but they are now 6 and 7 behind this new one and I have the feeling that Inno want to strengthen the guild interaction
So, for a guess, I'd say it produces/enhances guild medals (not done before, though Gaea was quite recent); provides population (not done since Contemporary); and I wouldn't be surprised if it gave a third bonus too (not seen since Colonial) - maybe troops.
Thing is, if it does three things it probably will not be tops at any of them, so will be slated as being a poor GB even if the combination pays well for those who want that combo.