Besides the obvious informational value, and the ability to rename, add, remove, and preview the pets, one of my favorite things about the Pet Journal is that it gives the game devs the creative ability and outlet that was somewhat taken away when pets became trainable spells. Pre-pet spells, a few special pet items would have flavor text on them. I've written about it before, but a great example of this would be the Firefly. "Still flying..." it used to read. *sniffles*
With the Pet Journal, I've noticed some pets have flavor text added to their "Info" section. For example, the Black Lamb's info reads, "Offspring of the black sheep, the black lamb is only found in the forests of Elwynn."
Ever since I discovered that, I've been checking each companion's Info section to see if there is any new flavor text. I'm not expecting every single companion to have something written there, as that would be overkill, but I love finding small surprises in the way of a newly added description that's amusing or informational. Those little details just show how much thought and tender care was put into the creation of the game and this particular feature.
My absolute favorite descriptive text so far is from a pet that won't even make it into the release of the expansion. Here's Jacob the Test Seagull's flavor text, in case you can't read it in the image below: "[DEBUG] This pet comes from the magical Test Land where it frolics all day in fields of structs." (Wikipedia: Struct - (short for "structure") is a computer science term for a record that is used to store more than one value.)
I can't help but wonder if all pets come from this magical Test Land, and when they "die" do they go back there? Instead of 'All Dogs Go to Heaven', is it 'All pets go to "magical Test Land"'? LOL :P
Anyway, I can only hope more unique and interesting flavor text is added for more companions in the journal. Especially if a particular pet has an interesting back story to it or is a nod to pop culture or something else.