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I watched this tutorial and they made a bedroom floor by creating a plane and adding thickness modifier.
I don't get why they would use that instead of just extruding it up along the z-axis?
Only difference I noticed is the thickness floor only has a top face on edit mode, but extruding it creates faces on all sides.
Does it matter for creating models?
Or as long as you don't need extra faces, then thickness would make sense?
Is there a benefit for using either or?
The reason is that extrude (red) makes a hole and you must create another face by hand, whereas solidify modifier (blue) doesn't. As long as your model has all the faces it makes no difference as you can't see what's inside. Still, when you remove some faces as in this example... 
The Solidify modifier gives the same result as the Extrude Faces Along Normals operation, except a modifier doesn't create any real new faces, so it allows you to change the topology and thickness more easily, and also, as Gorgious points it out, it makes the object lighter.
With the Solidify modifier you can give a different material to the extruded faces, you can even give its own material to the rims, but contrary to a real extruded object you can't unwrap these faces, so it's a limitation.
That said, for a floor, if you don't need to show its thickness, you don't need to extrude it or to give it a Solidify modifier, a simple plane is enough.
For an object like a shirt, the Solidify modifier is very useful as it gives thickness to your object without making it heavier, and you can easily rework the topology. You don't need to apply it, just put it after the Armature modifier if it's for an animated character.
