In the absence of strict procurement practices and robust record keeping its all to common to see organisations struggling to retrieve their records of purchase backing-up their claim to entitlements. In fact how often do we hear "yeah we've got 20 licenses for that - they're listed on Dave's spreadsheet".
Now lets be clear - the fact that it's on Dave's, or Susan's or anyone's spreadsheet does not constitute evidential fact. For that, you'll need the Proof of Entitlement if issued by the vendor, or the (signed) Contract containing the license grant, or the Order issued under it for the products in question. At a minimum if those are lost in the tracks of time (no doubt residing only in someone's email who has long departed the employ of the company), you'll need the latest invoice that shows the products and quantities that were covered by the last payment (ie. either actual purchase or renewal).
Again, its all too common that it's not until an audit that organisations are forced to scramble through the purchasing, legal, IT et al records looking for some artefact to substantiate the otherwise baseless right of use claim for the vast overage of licenses that have been deployed! This trek down memory lane can be the most time consuming - and often fruitless - use of specialized resources, the cost of which is not generally recognized by management and similarly overlooked in the justification of a dedicated SAM function.
So what's the alternative? Quite simply a process that ensures those essential records are properly recorded in an organized and readily accessible system, and are kept current through routine and ongoing ownership - once established this is not as much of an overhead as it might seem, and having all of that data at hand when challenged by a vendor can go a long way in underlining your disciplined approach and credibility in such a way that you'll be last on the next audit round list, if in fact on their radar at all.
Now this will no doubt rally those skeptics with their "wait - I just call my reseller and say give me a list of what we own" approach, and while this might offer some solace it doesn't necessarily constitute proof in the same way that last document of fact - the invoice - does. How's that? Well for one example think of step-up licenses that will be printed there for all to see, but what about the original license it is based on (and worse, what if that original license is actually still in use!), or those 'from-SA' uplifts that require unravelling potentially years of purchase history to properly determine entitlement. All best avoided by having a routine practice supported by a specialized system in the first place ...