While not needed to make the shamisen playable, a yubikake makes the shamisen much, much easier to play. The shamisen is highly polished and moving a bare hand on the polished sao (neck) immediately creates friction. As there’s only three strings, playing songs on shamisen often requires sliding up and down the sao (neck) frequently, and the friction buildup makes playing more difficult and can be very frustrating. The yubikake eliminates all friction and allows your hand to effortlessly slide across the sao. You can still play shamisen without it, but using it greatly improves your experience.