It allows more fuel into the cylinders by the vacuum sucking fuel through the primer which needs to be locked to seal the priming line. The PA28 primes directly into the cylinder on three pistons only, can't remember which. This makes the mixture too rich and you get rough running much the same way as if you climb too high without leaning.
Also, when you try to shut the engine down using the ICO, you may find enough fuel is being drawn into the cylinders to keep the engine running.
I have seen this one one more than one occasiion and it should be the first thing to think about if a pilot reports a higher than normal idle, rough running at power or an inability to shut down an engine on the ICO.