Why is that a "good reason" to do it?

Not really :) and I am not the OP. This article [1] provides a very good overview of the repository organization Google has and the reasons behind it.

I think the reason that this works for well Google is the amount of test automation that is in place which seems to be very good at flagging any errors due to dependency changes before it gets deployed. Not sure how many organizations have invested and have an automated test infrastructure like Google has built.

1. https://cacm.acm.org/magazines/2016/7/204032-why-google-stor...

It radically simplifies everything. Every commit is reproducible across the entire tool chain and ecosystem.

It makes the entire system kind of pure-functional and stateless/predictable. Everything from computing which tests you need to run, to who to blame when something breaks, to caching build artifacts, or even sharing workspaces with co-workers.

While this could be implemented with multiple repros underneath, it would add much complexity.

I think this is like the "your data isn't big enough for HDFS" argument earlier this week. The point I take away is that at some stage of your growth, this will be a logical decision. I don't think it implies that the same model works for your organization.

And Facebook.