It reduces instruction complexity and central neural system load. Your brain constantly makes calculation to errect musles in right way to keep body balanced.

Imagine if instead of left arm you would have 2m long tentacle which grows from stomack. In such situation center of mass during movement will be a mess. It would increase brain work immensivly just to keep you straight

Furthermore, tentacle and arm have completly different moveset and thus require different instruction set. That also increase load on brains as it cannot reuse same thing twice. It also would take huge extra memorry to remember all possible moves each limb can make.