Reason #1: To move better
In order for your body to be able to move properly, all the joints in your body need to be in alignment. When some of your muscles are much tighter or stronger than others, it throws your joints out of alignment. Think about it, if you have a machine that is turning because of a bunch of little gears, and one is not aligned perfectly with another, the machine stops working. Now I know many of us walk around all day out of alignment and still can do our daily tasks, but things will become much easier and much more comfortable if our machine (our body) is working in perfect sync with itself.
Reason #2: Pain
Reason #3: Strength
How can we be as strong as possible, if we can't move our body and limbs in a full range of motion? In order to get the maximum amount of gains out an exercise you need to be able to perform the entire movement. For example, if you're trying to shoulder press, but your shoulder can only go 3/4 of the way up because your chest is so tight it's internally rotating your shoulders forward, you're missing a quarter of the movement pattern. This is the same for having tight hips and ankles, and not being able to squat to parallel or even close to it. Not performing exercises with a full range of motion is cheating yourself out of strength and size gains you could be getting with proper mobility.
Reason #4: Injury
Lets look at the shoulder example from the previous reason. When your shoulders are pulled forward, you can't press all the way up. In order for your body to full extend your arms, your spine is going to have to compensate for the lack of range of motion. This will put your lower back in a compromised position and at risk for injury. When your chest is overly tight, it also pulls on your upper back and can cause back pain around your rhomboids. When your hip flexors are overly tight, as shown in the image in reason 2, it causes a lot of tension and pain in your lower back.
I hope this blog helped give you the motivation to do your mobility work everyday so you can be a complete human being. We weren't designed to sit in a chair all day, slumped over a keyboard. We were made to move, and unfortunately many of us can not do that properly. So close your computer, stand up, and go do something on your feet!
Stay strong!
-Nick