I’ve seen, again an again, deploying to Staging and integration testing in that Production-like environment together with the software of other teams, reveal problems that we did not saw in Dev, thus saving us from deploying into Production software that broke or, worse, corrupted the database.

This was certainly very important when I worked in environments such as Investment Banking where Production being down because of integration issues or, worse, sending bad data to other systems or the database having to be rolled back to the overnight backup, might mean the business losing millions of dollars.

It’s not a foolproof mechanism but it certainly catches most integration problems, which are often most of the problems in complex environments where multiple teams are responsible for multiple highly integrated software systems,

Granted, little teams doing small software systems in simple environments were their software has little or no integration with other software, can probably get away with not having a Staging environment if their Dev environments has the same setup as Production (same OS, same database and so on) since they’re going to have very little in the way of Integration problems with other people’s software.