fnj minnesota

I have a 1990 cherokee limited with a 6 cyl. For a while now I have been trying to get it to idle properly. I replaced the tps,the o2 sensor,I cleaned the IAC and the throttle body,until today I came back from ice fisfing out on the lake and decided to unplug the vacuum line for the EGR at the intake,then the idle picked up to about 600 RPM. It idled great! I put my finger over the port on the intake and it goes back down. I took the rubber elbow off the plastic line and tried to blow through it and found that it was plugged. Obviously I need to replace the plastic line with a new line to the EGR controller. Anyway my longwinded question is why does it idle better and generally run better with the port left open ?