The reason that they are listed is due to the fact that if you exchange them for a guildleve with a star and a (1) beside it, this will increase the awarding gil by 10%, no cumulative. Combine this with the ability to trade in up to four, while utilizing a new guildleve, will allow for better rewards. In example, if the new guildleve you have chosen has a 1000gil reward, by trading 4 more with a (1) beside it will render you 1400 gil! This is very beneficial when doing a new guildleve in your daily routine!
To get the bonus just remember that you need to spot a new one, accept it, then trade in previously complete ones for the new one.
Sweet! Thanks so much! That definitely makes more sense! What about when you go to accept new gls, I've noticed the ones I have already completed being displayed instead of complete new ones, is this just a chance to replay them in order to get a higher reward in the end?