Hike in- hike out wilderness trips require carrying litres of water.
Going to the shops or work doesn't. Larger water bottles can be heavy, take up limited hand bag space and can be excess to needs.
The 400ml Classic is perfectly suited for those daily life experiences that don't require clambering up an overgrown dry creek gully through waist high stinging nettles searching for fresh flowing water because somebody, not mentioning any names, Emma, did not check the recent rainfall in the area.
Fits easily in the side pockets of back packs and can be easily attached to prams with a carabiner clip. Great for kids.