It makes exercise easier. Weight Watchers magazine reported that reading a book while exercising helps motivate you to keep going even when you do not feel like it. Sometimes running on a treadmill for even five minutes can be rough, but if the protagonist of your book is about to get stabbed in the heart by an ancient Greek blade forged by Hephaestus, then you might be inclined to finish your routine. So exercising your mind while exercising your body is a good thing.