Being an Adobe employee, have you reviewed Experience League yet?
"List props are a setting applied to props that allow the variable to hold multiple values in the same hit. If this setting is not enabled, or if the wrong delimiter is used, the variable is treated as a single value."
IMPORTANT: List props are still subject to the 100-byte maximum length. List props are easier to hit this limit and be truncated, since they can contain multiple values. Consider using abbreviations or shortening values if you might hit this 100-byte limit.
I've seen list props used for capturing errors before.
One other "real world" example that I use is to track Authors (in our newspaper sites). Most articles are written by a single author, but some are a collaboration. When I track my authors in a list prop like this, it means that each individual author will collapse into their own row, rather than be subject the various combinations and order that the authors are passed.
Example 1 (non list prop):
author one,author two 467
author one 356
author two,author one 243
author two 146
Example 2 (list prop):
author one 1066
author two 726
This allows me to see the popularity of authors individually....
Now, I'm sure you may be wondering why I wouldn't just use one of my actual "list" dimensions (which allow for much more information to be passed)... but that is because we only have 3 of those... and I need them to capture much larger sets of data... We don't have too many author collaborations that will result in needing more than 100 characters, when we do.. yes we have issues.. but we have to accept that risk.
(not answering your question directly)
I am amused that an Adobe employee has to ask this in an end user forum like this. I would've expected Adobe to have some internal forums or support groups where employees from one product line can ask questions to those in another product line.
@jeff_bloomer & @Jennifer_Dungan have both provided some great examples, I'll throw in another one as well: Situations where you can have multiple settings applied at once, and you want to see which single setting is the most applied.
Let's say you have a product list page, and you have the option to apply multiple filters, and have an event fire "apply-filter" after selecting multiple options. Instead of having "new_home_blue" being set as the value, you will get the values of "new", "home", & "blue" all counted at the same time when using "_" as a delimiter. Also great for multiple categories on a blog post, multiple tags, etc.