So darts in particular had some nice features that added utility in D&D. Darts of seeking, darts of wounding, asp nest, and others could be found in low quantities, but provide a decisive advantage in some situations. Darts were more expensive then a sack full of sling stones, and virtually useless against some types of enemies, but when they shined, they shined bright. Slings on the other hand, could themselves be magic items, that could use magical munitions. Slings of seeking were quite a prized item for clerics that didn't favor a crossbow, or spell hurlers who couldn't afford to constantly replenish darts.
These have always been available in D&D CRPGs, and they would be missed if they weren't added to BG3.
