What is the essence of collecting? To bring together different more or less thematically connected items is just one part of it. The more relevant and more distinguishing part is to preserve and carry over time these items and make the art, the ideas, the craftmanship, the beauty (and whatever other interesting aspect the collector sees in his collection) available to future generations -- Plus Ultra.
Now, why is this the more distinguishing aspect of collections? Simply speaking, because there are collections that don't make a big difference and there are collections that change the cultural heritage completely -- the first being not to say irrelevant but only important for the collector and the latter being important collections in the essence of the word. What distinguishes both collections is of course the question of what is collected.
If there are many other potential collectors taking in the collectibles although at a slightly lower price, then the only difference made by additions to such a collection is the (slight) price increase -- but for the heritage or the preservation of this item it is irrelevant to which collection it goes.
Now, there are different collections and these are collections that really make a difference: If there is no other potential collector that would take in these items or even worse, the only other one interested would take these items to the smelter because of its high material value, then such a collection that preserves them and takes these items over time is an important collection.
Frankly speaking, a vintage steel Rolex collection or a composition of some Patek Nautilus' is not so relevant in the most important aspect of collecting as a collection of unusual and out of fashion gold cufflinks or unpopular silver vases.