I mean, you still need to put the definition! Am I in the wrong here?

Custom blocks are useful.
First of all, you can stop reusing code. An example is when you are repeatedly using the same giant chunk of code, and the custom block can make the scripts WAY more tidier. Also, the input value is very helpful. For example, you are creating a platformer and want to animate a platform. That will take up a custom block. Only, the fact that you need input values for the things that can change the meaning of the custom block. Have i explained enough now?

And the extra speed of the ‘no screen refresh’ option when appropriate makes a huge difference. Try doing something like this https://scratch.mit.edu/projects/55182496/ without it.

custom blocks are good
i mean, it stops you having to reuse scripts eg.

could become:

Ok, thanks!