The usual, standard equipment of the kit is a kick drum, snare drum, hi-hat, 2 cymbals, and 3 toms. This is quite enough to keep the rhythm and make the fills. Choose the best hi hat clutch.
Extended equipment may include 2-3 or more volumes, additional cymbals, cowbell. And this option is for those who need full stuffing or the sound of different volumes in Latin American music. This is also understandable.
BUT WHY DUPLICATE TOOLS?
For example, 2 snare drums. Or 2 cowbells. Or 2 identical plates that are placed on the left and right?
The point is this, as Alexander Fedotov explains. This kit helps to avoid crossing hands when playing. Especially if someone uses a paradiddle, then his fingering will be such that the hands do not cross.
For more details, see videos.
To be honest, my opinion is optional. 2 snare drums – this is still understandable because the sound is different, one gives a deep, the other – less and in certain music – this should be used. But to duplicate almost the entire installation is too much. And carrying such a unit with you is unprofitable.