Andrey
Typically, the need for it comes after a fresh import of new photos from a camera or phone. There can be tens or hundreds of photos each time.
I would then go through them and mark any photos I particularly like for various reasons. Sometimes, this process can span a couple of days and having some indication of the last photo I marked helps me start near where I last left off.
2a. Photos worth post processing. (e.g. for Printing)
2b. Photo to be shared.
2c. Photo good for a photo album.
When I am done marking these photos. It allows me to filter them for my next actions.
When post process - I can filter to all the favorites and work on them as needed.
For sharing and albums - I can make sure I include the favorited ones along with any other photos that may be relevant to the album or the people I am sharing to.
Other workflows - I may need to choose from the best of the favorites when printing an album.
Essentially, I use it as a way to identify top photos before any filtering happens. This is useful because it also allows me to spot any mistakes where a photo may have been left out. This is especially helpful when culling family or trip photos where I want to tell a story and there may consider bringing in one or two photos in between two favorited photos to ensure there is progression in the story.
