Contracts are needed for your fleets to be eligible to attend expeditions.
Each fleet has the number of workers that you have entered to it, contracts must be paid to each of the workers that are in your fleet, your fleets can be hired for a minimum of 7 days, but you can also hire them for 15 or even 30 days, once their contract expires, you must hire them again if you want to be able to go on expeditions.
It is not possible to dismantle fleets with an active contract.
When trying to add workers to a fleet with an active contract, you will have to pay the equivalent of the available contract days of the fleet to be able to join it. For example, if you have a fleet with 3 active contract days, and you add a new worker, this worker will have to pay 0.5 USD + 3 USD (3 contract days) in $ETERNAL to join that fleet and be able to mine.
The cost of hiring your fleet is 1$ for each worker in it, obtaining a small benefit if you hire it for more days: