Paper charts will show you a wealth of knowledge, all condensed onto a single page. While paper charts will show you everything you could wish to know about depths, bottom conditions, and restricted areas; they are not absolutely necessary on a cruising sailboat.
Paper charts are rarely found on cruising boats because electronic charts seem so much easier to use. A paper chart shows you a big picture as well as a detailed view of the world you are sailing in, where an electronic chart plotter shows you the world you are sailing in as well as your position at that very moment. Electronic chart plotters are very convenient, but the screen alters your view of the world.
On a paper chart you can view everything in your area at a glance. An electronic chart plotter is restricted to the size of the screen. Zooming out to view the big picture takes away all the details. Worse yet, some charted markings do not show up at certain zoom scales. This means you could run into a charted obstruction simply because you had the screen on the wrong zoom setting. This occurred during the 2014-2015 Volvo Ocean Race, when Team Vestas Wind collided with a reef because it didn't show up on their electronic chart at the zoom scale it was set to. This collision could have been avoided if they were using paper charts, as it would have been a visible obstruction on any scale.
Lastly, if your electronic chart plotter were to fail, having paper charts would still provide you the navigational data you need to safely sail to your destination. Paper charts do not require electricity or connectivity to an antenna, they simply show you the information and keep you informed while you cruise. While paper charts are not mandatory to carry on a cruising sailboat, they are a valuable resource and accessory.