Having a K-ring set (or - preferably - several) is akin to fooling around with Lego bricks. For adults of course. But notwithstanding that caveat, they are so versatile and can be used for a surprising number of photographic applications. Extension tubes, spacers for allowing bellows to mount on modern DSLRs, acting as sun shades, makeshift adaptors for exotic lenses, quick-release filter holders (K2 plus K3), getting close-ups for wide angle lenses (Nikkor 20/3.5 + K-1 is a classic), etc. etc.
I must have acquired around 25-30 of these sets over the years.