Leah is kinda meant to be an official, overall Rabbi for Starfleet itself. She stays at HQ and, for Jewish crew members on the Enterprise
or the Columbia
or anywhere else, they can contact her for spiritual guidance or just to talk (hence the line about her being a person that some crew members could go to if they didn't want to bother Phlox. The others (the Imam, etc.) would fulfill similar roles for crewmen of other faiths.
In another story I have here, The Light
, she's contacted when a crew member has a death in the family, and she guides him through conducting a small service.
Thank you very much for reading, and for your kind review. So many times, I see people treating the MU as a big, campy lark. To me, it's tarted-up totalitarianism, and Hoshi is the biggest tart out there.