You fold them so you can stack them on other folded things. If they're messily folded, the pile falls over as you carry the basket upstairs, or stack other things onto them on your shelves.
For me bedsheets are their own wash cycle, so there’s nothing to stack them with. I just carry them upstairs and dump them into the blanket box when they're done. No need for a basket.
But let's say your washing process is different to mine. I'd still rather do two trips upstairs than fold sheets.
