Natural Selection
For those who have it, the visible tendon connects to the palmaris longus. There isn't any real reason that the muscle exists—research notes that its presence in our forearm has no bearing on arm or grip strength. In fact, it's so useless that surgeons will often remove the muscle and repurpose it for reconstructive or plastic surgery operations. The palmaris longus is one of many strange, ancient relics that remained a part of our bodies because it was too inconsequential to have totally disappeared.