I just finshed building my first PC and there are tons of leftover cables. A lot from the case and a few from the motherboard. The case is a NZXT s340 Elite, and the motherboard is the Asus ROG Strix z370-e gaming.
Like what kind of cables? The small case cables for the front panel power button and audio go on the motherboard, and the USB 3.0 goes on the motherboard.