I would greatly appreciate having this feature.
Use case from me:
I would create multiple diaries such as Daily, Relationship, Family, Work, Travels, Places I want to go. And another diary as a catch all. I have a money tracker app and I was able to filter quickly all the transactions relating to the "book" and view the overall status of the "book".

With multiple diaries and advanced filter feature, I would also hope to be able to see entries only relating to these diaries. Eg. When I am selecting Family diary, I would only want to see Family related entries in all the tabs (i.e atlas map, calendar, timeline, etc). This is so that I can have better view of where I visited with my family, or where I visited with my partner etc instead of seeing everything in the map.
The search filter only filters at list view. But as a visual person, having the ability to filter only the related entries at atlas map would be tremendously helpful.
In summary, while the app have the ability to filter by tags, it would be helpful to group the entries by diaries ("categories"). I can reduce the need of tags eg from 2 to 1 (#DailyReflection, #WorkReflection into #Reflection).