In this specific case, they do test for the ApoE gene which can you a 20x higher chance of getting Alzheimer's https://en.wikipedia.org/wiki/Apolipoprotein_E#Alzheimer's_d...

20x more is not a sure thing, but for a disease like Alzheimer's that is already pretty common (e.g. we're not talking about risk increasing from 0.01% to 0.2%) it's more meaningful than just for entertainment purposes.

I agree with your wife that for most people their 23andme results will be nothing more than entertainment, but there are a handful of truly meaningful things it can find.