You can look at piece value as a heuristic for the strength of your position. The problem with purely positional play without regard for material is that you co

You can look at piece value as a heuristic for the strength of your position. The problem with purely positional play without regard for material is that you could miss some tactic that forces exchanges and suddenly you're down three pawns and losing. As humans we miss tactics a lot, which is probably why we value material as much as positional advantages.