The world’s becoming a stingy place, because no one’s respected if he isn’t rich. But I attach little value to wealth – it does no one any good: a curse on worldly riches. But I’m supposed to be telling of Perceval!
Gerbert of Montreuil, Gerbert’s Continuation of Perceval
Riches are a substitute for Fame; a token1 you gain from being wealthy2, and can place and shuffle around your Motifs3 to spend on a 1-1 basis whenever you’d be invoking or spending Fame.

It’s not a perfect match: when you invoke Riches, you never get to keep them. Silver has a way of slipping through your fingers. It’s not durable the way Fame can be.
Nor does it count as Fame for the purposes of having Motifs In Doubt. Beware that gold, however glittery, is a poor substitute for iron.
Spending Money?
To be clear, all knights are filthy rich. You can be poor and desperate from your class’ perspective while still having the kinds of resources most common folk will not see in a year. Fame and Riches both can augment that if you’re making a particularly tough purchase, but this can be somewhat loose because of the gift and favors economy most of the world operates on4.
Footnotes
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My go-to would be replica Medieval silver pennies. ↩
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Either from your Estates and other titles (i.e. you’re just a rich person) or sometimes you might get Riches as a reward for some adventure (e.g. if you unearth a treasure chest). ↩
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Either representing immediately spending money, or that you had spent money previously, and that’s why your gear is better, your companions so eager to support you, etc. ↩
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i.e. you’re not usually buying many of these things directly. You’re being gifted them by someone else, but with the expectation that your estate has sent gifts before, or will be sending gifts afterwards, and the overall handling of affairs will be balanced on a kind of honors system (that nobody is supposed to be strictly balancing, but everyone is). ↩