Being sick is just no fun. If you have to spend a day in pajamas on the couch, it is far more enjoyable while healthy and curled up with a good book.
There is increasing evidence that meditation and mindfulness can help with boosting your immune system, through retraining your brain and the CD-4 cells (the part of your brain that assists with immune development).
Meditation is also a very effective way to reduce stress in your life, and the less stressed that you are, the lower the likelihood of getting sick.