[oftenwrong]

To put it in the most general terms: It provides the same value that using a VCS has for a project, but applied to the entire company.

In a standalone project, would you accept a change that is incompatible with other code in the project? For example, would you allow a colleague to change a function in a way that breaks the call sites? No, you probably would not.

The attitude within monorepo shops is that this level of rigour should be applied to the entire company. Nobody should be able to make a change anywhere if it would break anything elsewhere, or they should only be permitted to do so with intention. There are caveats to this, but that is the general idea.