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One of the prevailing assumptions that fans of Continuous Integration have is that builds should be reproducible. By this we mean that at any point you should be able to take some older version of the system that you are working on and build it from source in exactly the same way as you did then.
This isn't called out as a key practices in the sources I usually refer to on the build process. I think that's because it's an underlying assumption - one that's considered so obvious there's no need to explain it.
One of the driving reasons to have reproducible builds is to ensure we can deal with problems in past releases that are still used. If we release software to a customer a year ago, and they now report a serious bug with it, it's important to be able to recreate that software so that we can deliver a fix.
But let's assume a case where you're releasing software every week to a hosted environment. Let's also assume you have a solid Continuous Delivery process and are thus confident that you promulgate bug fix by either waiting until the next release or(if really critical) doing an early release. Do you then still need reproducible builds?
In a scenario where you: receive a bug report, reproduce the bug on head, fix it on head and either wait or immediately release - then you don't. But there are cases when it's still very handy to have reproducible builds.
What happens when you get a bug report and you can't reproduce it. Do you just declare it fixed and move in? I wouldn't be happy with that response. Firstly I'd want to be sure I really understood the bug - so I'd want to check out the released version of the software, build it, and ensure I could then reproduce it. To be confident in reproducing the bug, I'd need to reproduce the build. Furthermore even if I'm confident that the bug got fixed en passant during recent development, I'd still argue that there's at least one test missing. I'd want to write that test and verify that it passes now and fails against the released build.
Another case is a regression. A customer contacts you and says there's a bug now that wasn't there before. Such bugs can hide a long time before they wake up and wave their feelers at you. Maybe it only occurs when the first of the month falls on a Monday. Either way you now have software that you think worked two months ago but now has a bug.
Here having reproducible builds gives you the ability to use DiffDebugging. Your customer is pretty sure that you didn't have this problem two months ago, that was build 20000, you're now on build 28000. So you check out build 20000 and look to see if the bug is there. It isn't so you try build 24000, not there either, so next is 26000. Before long you know the bug first appeared with revision 26543 (modern version control systems have features to help you do this). Now you look at the diffs between revision 26543 and its parent - often this approach makes it much easier to find a bug.