The minimal second option earns its weight for one specific reason: communal showers. That alone justifies it, and everything else it does is a bonus.
Something flat and thin, a few hundred grams, packs flat against the back panel. It also means your main shoes get a night off to dry properly, which extends how long you can go before they start to smell.
Not a second pair of shoes. A fallback, and the distinction is what keeps the weight down.