Actually, partitions aren't recommended either. It won't do any harm, except restrict free space usage, but there's absolutely nothing to gain either.
In the old days, when we had spinning drives with only one physical read/write head, the advice was to put scratch disk on another physical drive (not partition). That recommendation has stuck around to this day, but with SSDs there's no reason for it anymore. The official recommendation today, for best performance, is to put it on the system drive (if you have enough space).