The difference is the currency value due to demand and recognition as a legal tender. It's a token of recognition that other parties do need your goods and services and hence are backing your currency. You can ask yourself a similar question to get an intuition about it: what's the difference between IRS accepting payments in Bitcoins or dollars vs accepting only dollars?

As an economy noob I would like to understand this as well. Is Russia maybe impaired in some way from propping up the ruble using USD due to the sanctions, and this would get around that somehow? Or this makes no sense?

Its a fuck you move.