Ah, the importance of documentation!! Sitting on my desk in front of me is a BGS Sumpftarn hat. I could easily see someone thinking it is worth $10 if they don't know what it is. I can see them being thrilled if someone offered them $30. That person might even think they are offering a fair price as no collector can be an expert on the value of all things all the time. However, it is worth minimum $100 on eBay consistently over the past few years.
When someone is trying to liquidate all the assets a person has accumulated, everything from that Sumpftarn hat to furniture to garden tools, they are going to be overwhelmed and basically be prone to taking way less for the stuff than they could otherwise get. A spreadsheet or database that documents and estimates the value of everything would at least give the executor some gauge to know if an offer is "good enough" or "insultingly low".
If the collection should go out to bid for other collectors, the inventory list keeps potential bidders on the same page. It also gives the bidder more assurances that bidding higher is worth doing. Someone calling me up saying "I have a storage unit's worth of military stuff, how much will you give me for it based on these 10 bad photos?" is going to get a much, much, much lower bid from me than someone that has the same locker's worth of stuff and an inventory sheet.
Lastly, if you are one of those collectors that has a wildly diverse collection, do your heirs a favor and group the stuff according to collector groups. I'm a camo guy so if someone has a collection of camo and WW2 badging, I'm not going to offer much of anything for the WW2 badging because I don't want it nor do I really know what to do with it. So the heirs, if properly educated, could know to offer me X and offer someone else Y (as Y is likely not interested in the stuff I am).