I saw the question and my instinct was 'poll', but I read the comments and decided to wait and see. I agreed with your comment that the questioner was asking for concrete examples, so I figured it's worth a shot to see what kind of answers the question garners.
My tendency was to agree with Anthony Pegram:
This question is "create an arbitrary problem and then a solution
where Linq is useful." This is not an effective SO question. We
provide solutions to problems, not ideas for problems that then need a
solution. This is a poll, not a question with a single best answer.
However, having read the answers I don't believe that this is a constructive question. Asking for 'examples' of something leads to both duplicate answers (the obvious
SelectMany) and incorrect answers (i.e. offtopic, e.g. the
GroupBy answer which the questioner notes he already grasps).
The question is too close to being a simple What's good about LINQ? type poll question. I don't understand whether it's serious or just an attempt at making a popular question.
people say you can avoid heavy nested loops by using LINQ, but few LINQ examples come with pre-linq code listings
If the questioner had googled, there are plenty of examples of 'pre-linq code'.
These two, for example, deal with actual problems.
If the questioner had an actual problem then they should ask that question. As it stands, it's just an attempt at starting a discussion.