What is dynamic batching, and why is it off by default?

While reading this tutorial, I came to a section where it said something like “as you can see, dynamic batching has stopped working”. I did not see that. In the image in the tutorial “Saved by batching” on the stats panel had dropped to 0, but my stats panel still showed a number in the 1000s.

When I checked the player settings, I noticed dynamic batching was turned off. After a bit of investigation, I learned that dynamic batching is now off by default for 3d projects as of Unity 2018.1; but why is this useful?

A mesh is the part of a 3D object that describes its shape. When certain criteria are met, Unity is able to combine meshes from different GameObjects into one big mesh before they are drawn. This is called dynamic batching. The "dynamic" part means it happens while the game is running.

Sometimes (but not always!), it takes less time to draw one big mesh than it would to draw lots of smaller meshes. Dynamic batching occurs on the CPU, and drawing meshes occurs on the GPU. You can think of dynamic batching as spending CPU time to save GPU time. This means that it becomes more likely that the game becomes CPU-bound (meaning that the frame rate is lower than you want because the CPU is taking the most time), but you should ideally want as much as possible to happen on the GPU; that’s what it’s there for.

If your game is already CPU-bound, then dynamic batching will not help. But if your game is GPU-bound (meaning that the frame rate is lower than you want because the GPU is taking the most time), then it may help.

(FYI: The GPU equivalent of dynamic batching is GPU instancing.)

For these reasons, dynamic batching is desirable for old mobile devices that don’t have a GPU (or have a bad GPU), but it is not efficient for games intended for more modern devices.

Here is a helpful article that goes into a bit more detail: Why Are My Batches (Draw Calls) So High?
Also: https://docs.unity3d.com/Manual/DrawCallBatching.html