Here's the history of DLDI.
Back when the R4DS first came to be, along with other existing flashcarts, all homebrew that used file access, things like saving and loading data, needed to be patched with DLDI.
Each individual cart had its own DLDI in place.
Eventually the R4DS implemented a feature called auto DLDI, meaning patching was no longer necessary for file access on homebrew.
Auto-DLDI has been a stable feature of all flashcarts ever since, it is extremely rare that you'll need to DLDI patch anything now.