In practice
Keeping the two apart brings order to a trade. The setup is the context that makes a trade worth considering, such as trend, location and time of day. The trigger is the precise event that says now, such as a candle closing beyond a level.
With both written down, a trader knows what to wait for and when to act, and can review each part separately afterwards.
What to watch for
A trigger without a setup is just a pattern in a random place. A setup without a trigger is a prediction. Most poor trades are missing one of the two.