A procedure block that gets the level of a status effect can be useful to make custom potions act differently based on their level (like vanilla ones) or simply to check how much levels of a specific effect is the player affected by.
Issue comments
You can't get amplifier of vanilla effect.
You can use this block to access any dependencies provided by triggers you can't access directly. You could use it to access say x coordinate by typing x in it, but x is already a standalone block. When selecting procedure for a trigger, you can see the list of dependencies and you can access all of them using these blocks.
You have a dependency called amplifier in custom potion procedures that reflects the power/level of the potion. You can use this dependency to get the level of the potion.
To use this dependency, use Number dependency from Advanced tab and enter amplifier in the dependencyName box.