Why? Four reasons:

1) Going hard all the time never, ever, works
If you try to go hard every single workout, week after week and month after month, you'll end up with a mix of serious and half-assed workouts, and if you don't get hurt, you'll probably burn out completely at some point.
By the end of any given training year, you'll discover you would've been better off taking planned breaks, rather than letting your body and brain decide when you're ready to push toward a peak and when you're not.

2) Your muscles and joints need a break
Not every part of your body recovers at the same pace. You can restore energy substrates in your muscles faster than you can remodel tissue that's been damaged from serious training. Muscles repair themselves faster than connective tissues. And connective tissues might be ready for a serious workout before your central nervous system has fully recovered.

3) Sometimes you get stronger by not training
With full recovery comes supercompensation. With supercompensation come greater gains in size and strength and higher levels of fitness and conditioning. This is why swimmers and runners taper before major competitions in which they hope to break records, and why a lot of serious lifters will describe how they hit PRs right after a deloading phase.
A review published in the NSCA's Strength and Conditioning Journal compiled this amazing list of benefits that research has attributed to tapering:
• Up to 20% increases in strength and power
• Increases in muscle cross-sectional area of 10 to 25%
• Lower levels of stress hormones
• Higher levels of Testosterone
• Better moods during the day, and better sleep at night

4) Training is a marathon, not a sprint
Finally, unloading is just plain healthy, no matter what your age. Along with pampering the musculoskeletal and nervous systems, a phase of relatively easy training is also good for the immune system. Train too hard for too long, and you'll not only feel tired and unmotivated, but you'll also increase your risk of catching a cold or flu.
Since a nasty case of the stomach flu can torpedo any program, in the long run you'll rack up more workouts, and better workouts, by giving your body a break from time to time.