What it teaches
Choosing between rz and ż. Unlike ó and u, this one has a rule a child can apply on sight: after certain consonants it is always rz — przyjaciel, krzesło, drzewo.
The second useful group is the -arz ending on names of people and trades: lekarz, malarz, piekarz. Between them the two groups cover most of what a child of this age writes.
What to look for
Say the consonant before the gap out loud together. If it is one of the nine, the answer is settled without any need to remember the word.