First off, you only update the environment when there's an change to the environment. For example, if a module's required version goes up. Or, if you now need a new module. Or, and this is kinda weird until you think about it, you no longer need a module. As for why, there are several reasons.

• To make sure that each environment version is completely self-contained.
• To make sure that each environment version is correctly built against itself.

But, the most important reason is that you have a "last known good" to fall back to when something breaks. This means that you don't alter an environment after it's been deployed against. Otherwise, you don't have anything stable.