Why? I'm not sure but I think it's a number of reasons for me;
1.) My first real knitting project.
2.) You will not get this done quickly so it's pretty much a side project.
3.) It can easily fit in whatever bag you carry so you can do it while waiting in the doctor's office, on a bus, on a plane, etc.
4.) The cuteness factor and how puffy they are.
5.) I also figured out a crocheted version for myself too.
6.) You can attach all the puffs together for a quilt or whatever you can think of! I have already suggested lining the walls with them.
7.) Tons of swaps are appearing on Ravelry for puffs or even left over sock yarn.
The puffs I have made so far. A few I have crocheted too and the purple ones was from a friend after I traded mine too. I have already convinced a few people they should try it and it's rather interesting what everyone comes up with.The bag I am currently going to keep my puffs in.