The Kirkpatrick model helps answer whether an effort works. Level 1 is reaction: Did the participants like it? Level 2 is learning: Did they learn it? Level 3 is behavior: Do they use it on the job? Level 4 is results: Does it make a difference for the company?
Most only measure the first two levels, because they are easy. But it is levels 3 and 4 that show the value.
Build evaluation in from the start. If the learning objective is clear, it is easier to see what should change and how it can be measured.