Borrowing from libraries sort of gives money to the creators, since the libraries seem to buy + dump lots of copies. Buying used gives the libraries money, since they dump the copies at the used store.

Alternatively, be like me, and have a $100 budget for N ~= 20 BluRays/40 DVDs. 2-3 can be new purchases if used isn't in stock.

Lip service to the law, in theory purchasing used discs supports original sales of new discs, and I at least sometimes prefer the discovery experience of physical browsing.