Why do some countries separate a transition level and a transition altitude?
By definition, the transition level is the lowest usable flight level above the transition altitude. So if there is a transition altitude, there is a transition level. 

In the USA, the transition altitude is 18 000ft. If the QNH (altimeter setting) is higher than 1013.25 mbar then the transition level is FL180. This provides at least 1000 ft separation from aircraft flying at 17 000 ft (or 500 ft from the VFR at 17 500 ft).

If the QNH is lower than 1013.25 mbar then the transition level is FL190 or even FL200 to ensure there is at least 1000 ft separation from 17 000 ft. 

The transition level in other countries is based on similar logic.