The "modern usefulness" of a game is, essentially, an arbitrary mark from 0 (not useful at all) to 10 (very useful). The modern usefulness is something of a wild card in the valuation formula and it exists essentially as to reduce the valuation applied for items which are not, objectively speaking, particularly useful. For example, a spellchecking utility for the Spectrum 16K would be considered not useful, as it's "not useful to the vast majority of people". Although it might be a very hard-to-find item, if it's not useful to the vast majority of people, it will be valued low.
On the contrary, if a game has 'suffered little' over the years and is as playable today as it always was, then it will be considered very useful, as it's "very useful to the vast majority of people". This will mean it retains its value.
Again, with 161,456 items (and counting!) on our site, we haven't been able to accurately work out the "modern usefulness" of every item - but the form below will illustrate the concept for the ones we have assessed. Choose a Machine Type, a format and the name of a game. The base price already uplifted by the % Chance Of Being Found and General Esteem will then be reported, along with the calculation applied for the Modern Usefulness value...
That's not the final valuation - that's the valuation when both % Chance Of Being Found, % General Esteem and Modern Usefulness are applied.