Can someone explain the benefit of using a separate env for each project? Why not just use the default env for all your projects? Does it have something to do with the ops part of devops?
If you just want a sandbox area where you'll be developing lots of little scripts and bits and pieces, there's nothing stopping you from having a single environment that you keep adding to as you go. You just miss out on the portability aspects mentioned above and potentially risk getting conflicts over time as the number of packages you install grows and grows.
Keeping your project dependencies isolated also encourages you to document your dependencies (e.g. in requirements.txt), which is essential for reproducibility. If you have every package from every project installed globally then you tend to end up having issues when you or someone else tries to install a project in a different environment (the classic “I don’t know what’s wrong on your machine but it works fine on my mine”).