People going to a city they do not know may want to plan which streets to take to visit different places. There are many ways they can do this.
They can draw on top of the map which streets to take. That way it would be really easy to know how to continue.
If the map needs to be used to go to many places, all the lines drawn may end up being too many lines. Instead they can write down what street to take and where to turn, and along the map they can get the same information they had in the previous case, although it will be harder to see.
If they cannot take the map with them, for example if they have to return it, they may also want to write down some other useful information like how far are the streets from each other, and maybe even draw a small copy of the map. This time, they do not have all the information, but they have enough.
By compressing we change its representation to be smaller. With a lossless compression we can get the exact same thing we had in the first place, but with a lossy compression we will not get as much, but it might be enough our needs.