what is the point of telling add_filter how many parameters you want passed to the function?

The reason is that you can register native PHP functions as callbacks. Passing more parameters to them then they expect will raise a warning.

Imagine a filter that can pass 6 parameters and trim() as callback. PHP would now raise an error:

Warning: Wrong parameter count for trim() in …

Some custom callbacks might also change their behavior depending on the amount of passed arguments. That’s surely not good code, but such code exists, and it would break if WP would change its current behavior now.

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