An acciaccatura doesn’t occupy any “written time value” in terms of counting the beats in the score, but when it is played, it takes up some “real time” which has to come from somewhere. In the middle of a score, that time is taken away from the previous beat, but right at the start there isn’t a previous beat.
If you want to export audio and synchronize it with something else (for example, use it on a movie soundtrack) you need to know exactly where the exported audio track starts, and by default that is on the first beat of the first bar.
Since an acciaccatura is played before the beat, you have to move the start point back to get the grace note to play.
The playback duration of the grace note itself depends on other parameters in Dorico (which you might not be able to change in Elements) so “the start of the grace note” is not a very well-defined point in time, but setting the pre-roll to say “0.5 seconds before the first beat” is well defined and you can still line up the audio track exactly, allowing for the 0.5 second time offset.
You can work round this for playback in Elements by inserting an empty bar at the start of the score.