An accident upstairs is not automatically their fault
If a supply line fails in the unit above and ruins your belongings, your neighbor is only liable if they were negligent. A pipe that simply failed usually is not negligence. That means your own policy pays your loss, subject to your deductible, and then decides whether pursuing anyone is worthwhile.
The building policy does nothing for your things
The owner insures the structure and will dry and repair the unit. Your furniture, electronics and clothing are not part of that, and in a multi-unit building this is the most frequent way tenants discover it.
Loss of use applies even for a short displacement
A water loss can make a unit uninhabitable for days or weeks while it dries out. That cost is covered under loss of use, and it is the part of a modest claim that most often exceeds the value of the belongings lost.