Rather than asking that, what if we asked this question:
Why is it important to be able to move freely?
When this question is asked, the answer becomes obvious. Being more mobile means being able to move more freely.
It means becoming stronger and more stable while reducing the risk of injury and the effort needed to move.
To illustrate this further, imagine yourself that lucky person who grabs that shopping cart with the one wobbly wheel. We've all experienced this at one time or another. It is harder to push, sometimes harder to keep straight, and rattles the entire cart. You may even have pushed it just slower than you normally would so that you wouldn't draw attention.
Depending on how severe the problem is, you may have just tolerated it and continued on shopping with it. But over time, as more customers like you tolerate this wheel, the worse the wobble gets for this shopping cart.
Eventually, the wheel wears unevenly, the ball-bearings break and the whole wheel assembly requires a complete replacement. Had this shopping cart been regularly maintained, it may have lasted longer.
Your Body Constantly Adapts
Our bodies are naturally extremely good at adapting to conditions we expose it to. This is what helped us survive when we needed to run away from sabre-toothed tigers or hunt other animals for food. This adaptation is great when it comes to strength training as our bodies get stronger when we force it to adapt to increasing resistance.
However, we also need to recognize that the body also adapts in negative ways when exposed to "idle" conditions. This means that if you sit at a desk all day and all you push around is a mouse, it will start turning off muscles likes your glutes and core. Your hip flexors with shorten which in turn cause your hip joints to get accustomed to a reduced range of motion.
Your body does this to preserve energy it doesn't need to expend. Prolonged exposure to these idle states without counteracting it with intentional strength and mobility training will manifest into aches and pains in your lower back and hips. If we do nothing and repetitively subject our bodies to the same conditions, our bodies continue to adapt in ways that we don't want it to.
Summary
Mobility exercises and movements should be a critical part of your overall exercise program. This means choosing mobility exercises that focus on improving range of motion, stability and muscle control. It also means addressing areas of weakness and muscle imbalances, which when done purposefully will protect your body from injury. Ultimately, the goal of mobility is to help you move in a better, more controlled way without being compromised or limited.