I have two of almost every eye brush like 239, 242, 252, 222, 224,217, etc because I use them for different colors or products like others have mentioned. 217 is one of my fav blender brushes but it is awesome for concealer and dark paint pots. 239 I could easily own more because when you fade colors horizontally over the lid it is really nice to have lots of them handy so colors don't get muddy. 242 I use wet, dry and to smudge across the the lower lash line.
Also this might just be a ridiculous justification but I feel like having multiples keeps me from constantly washing them and I feel like the life of the brush is longer. It's sortof like wearing the same shoes everyday...they don't hold up that well. It's better to rotate!
Plus don't even ask how many angle brushes I have, seriously it's about a dozen!