for me it is that gardening provides me with yummy, clean, inexpensive food in exchange for useful exercise. i can't get into going to the gym or monotinous walking up and down the road - my body requires some kind of work which engages the whole of the spine, digging and gardening does that. the added parts are the tactile and visual feedback when shelling beans and sorting them and pondering what might happen next.
and then there's the scientific aspects of gardening where you can experiment and test things out and learn more and watch how the animals and insects respond and ...
well, i can go on, and on...

pretty colors, shapes.
if you are into beachcombing for rocks beans can provide some of those same things even if you live a ways away from any beaches or suitable streams.
in the middle of winter being able to run my hands through piles of different colors and shapes of beans and having things to sort out and enjoy is just very satisfying. OCD aspects for sure.
and then also the major bonus of being able to get new beans as pretty as Tinker's Fire or some of the other's i'm seeing now. after years of planting and hoping and then *poof* all at once i have a bunch more candidates for edible and dry bean feasting. green, purple and wax beans are all among our most favorite garden vegetables - i like eating them almost any ways they're used but fresh is a whole other bump up in goodness.