It spells out a message in morse. No, I'm not going to tell you what it it.
In another workplace, the retrieval of log files from customer sites had become a paperwork nightmare until a little feature was added that meant shift-clicking the Help > About... button opened an 'admin console'. By the time I'd left there the 'admin console' had seven tabs worth of features.
For example, part of our application is controlling PTZ (pan tilt zoom) cameras, but we only have rudimentary manual control. I got tired of using our clunky buttons, so I built in gamepad support a few years ago, and it has become one of our sales team's favorite features. One of our developers didn't like the layout we had, so he built in a shortcut to change it, and a bit later a customer requested that and we were able to use it. We also have a map interface set to up being north, and one of our developers thought it would be cool to change that orientation, so they hacked it in. Shortly after, one of the sales members saw his different orientation and asked how to do it, and now some customers use that feature as well.
Basically, when we get tired of working on boring business use cases, we try to add in some quality-of-life improvements, and those tend to be favorites of our sales team (after the fact; they seem to always ignore our suggestions).
I've been meaning to add in some less useful Easter eggs, but I honestly don't care enough to build something that could get me into trouble (my boss hates video games, so putting Pac-Man or something into it would be crossing a line).
It's probably not common anymore due to trust and legal reasons.
[1] https://en.wikipedia.org/wiki/List_of_Easter_eggs_in_Microso...
