In short, the unwritten benefit can help you save money. To me, this is the most important benefit because it affects my bottom line.

Let's use a HKG-MXP return as an example. Departing on 11-May, returning on 17-May, HKG POS. At the moment, the outbound has I class and the return is selling D class. On top of my head, there are a few benefits:

1. If I call CS now, I can reserve the I and D class fare. I am already locked into the I-fare. Even when HKG-MXP is selling faster than Taylor Swift concert tickets, I am guaranteed a discount business ticket. Since individual fares are rarely re-batched, I don't have to think about the cost going up.
2. I can waitlist for cheaper fare classes for my MXP-HKG. If I-fare opens up for my return, I save a lot of money from not having to pay D-fare.
3. What if I am unsure whether I am going to Milan Fashion Week or meeting the pope that week? I can hold a set of tickets each for MXP and FCO.
4. I can see MXP-HKG on 20-May selling I-fares, I can call CS to amend the reservation without paying thousands of HKD. This reduces a lot of change fees.
5. I don't have to pay until online check-in.

This is a very simple scenario. If you get creative with the ITA matrix you'll discover another dimension of complexity. If you want to understand whether your fare would be repriced, you need to read the fare conditions. The agents can tell you the 8-digit fare basis.

So what are the problems?
Revenue loss. Why not squeeze an extra few thousand a year from change fees? Why not force you to buy D-fares instead of I?

The extreme example was a couple holding up all 6 F seats, paying for two J tickets, and trying to force an upgrade during check-in.