Working together on a project can lift quite some weight of your shoulders. You can divide tasks and you always have an extra pair of eyes watching. While creating maps, this can also be very convenient. Sometimes you created a great map, but you want your coworker to take a look at it. For (fact) checking, for editing it a bit further or maybe to even add a whole extra data set. He/she could log in to your account, but there are easier ways.