I had to learn this for myself too
Firstly the amount of bandwidth that the simple text takes on the downlink from the satellite would be negligible, and they seem to “squeeze” in between the rest of the content.
Why are they important? (This is my view only and not the view of the Outernet team)
APRS is a simple and ready made “Back Channel” where one can transmit an APRS message and it will be delivered to the Outernet receivers in minutes.
At a time when information may need to be sent to a remote location (without internet) during some emergency etc. quickly you can see the use there.
Because the APRS message can be sent from a remote location via another satellite or HF radio and “inserted” into the APRS backbone to get to the Outenet transmitter, it could be used to request particular information from somewhere without internet access, that may be wanted at some remote location.
The input to the APRS network is self-regulating due to the restricted access ( by Licensed Radio Amateurs) there is little risk of anything “unacceptable” turning up in the text
Currently the idea is to create some traffic so that the interfaces can be tested, so most of it will be trivial at the moment ( e.g. I have a beacon running in HF once an hour which may or may not make it to the APRS backbone and thus onto the Outernet)
I hope that helps