It strikes me that Grease Pencil is a really strange addition, as Toon Boom or Adobe Animate does what GP does far better. Animation files (though not in native form) can be imported and applied to 3d surfaces. Why include Grease Pencil? Nobody really gets Blender to do 2D animation, and very very few Blender modelers use it.
I shouldn't really answer here because I've never used grease pencil and have too little experience in 3D to have my own opinion about it. But I often hear that other software is better than Blender for certain functions Substance Painter is supposed to be better at materials, ZBrush is supposed to be better at sculpting and so on. There is certainly something to this, the software costs a lot of money and they are specialized in one area Blender is the Swiss army knife, it has many functions and does a lot of things right, but in some areas it is not as strong as other software or even better than others
BUT Blender costs us nothing and it grows with time. As I said, you are certainly right with your statement, but GP may be greatly expanded in 1.2 years. In the end, I would like to say that ultimately you should try everything out and decide for yourself which way/software is better for you. I like to watch “Speedchar” from time to time, a man who creates excellent characters. He switched from ZBrush to Blender. Just as an example
You'd be surprised how many people use grease pencil, everyone loves it. There's a full short movie made with it. I believe it was a passion project from a Mexican director? I think. But the question is to the contrary, why not have it, even if just a few people ended up using it. It's been maturing over the years and it's a great addition to the repertoire of tools Blender has. It's the cool thing with open source, lots of people do 2D art and Blender was lacking it, so they developed it.
Blender History? One of my favorite subjects. First, a little animation history. In the ancient times of the late 1900s(1970s to 2010s) Computer monitors where made with glass. Animation Directors and Technical Directors would make notes and show information on a monitor by drawing on the monitor with.....That's right a grease pencil. Now the Blender history. The Blender developers added a digital version of this and called it grease pencil. This was very helpful for people to work from all over the world thru the internet. Since Blender is community driven, the developers added some additional features to the grease pencil. It was still just an annotation tool, but people started using it for storyboarding and even animation. Then came the big 2.80 blender rewrite(It's debated as to if this was the second or third one.) This is when Grease Pencil became an actual 2D animation system. The old grease pencil is now just called the Annotation Tool. It has come a long way since 2018. So the answer to your question is, because the community wanted it. There was enough people who wanted it that the developers said Okay hold my beer and now we have it. I'd also like to point out that Grease Pencil is a major part of why Blender has been added to Korean and Japanese animation studios. Like Project Studio Q. To say that people aren't using it would be wrong.
Sorry, Just had to throw this link in. I Believe this was at Blender conference.***Edit*** Just watched the first few seconds and it clearly says Blender conference 2023
Inklines Across the Spider-Verse - Using Blender at Sony Imageworks
Hey Randall! It's a valid question. I myself used to ask the same about features like the VSE, or even sculpt, when applications such as DaVinci Resolve and Z-Brush existed. While Blender is a great tool for just modelling, texturing and rigging alone, there are points along the pipeline involving note taking, illustration, storyboarding, concept art and more.
Initially, Grease Pencil was just added for notes. You could pass files back and forth, scribble out some grease pencil notes, or sketch rough revisions for animators, send the file back, and more easily collaborate.
But from 2.80+ it became its own dedicated module which could handle 2D illustration and animation, and it's gotten quite a bit of a following! There are of course applications which do 2D work better. There is no argument there. They do it better for a single reason: that is the specific task that they were designed to do. In a working pipeline, departments have to liaise and collaborate, share files, and agree on certain standards. In a small studio, it is quite possible to complete a project using ONLY Blender, and take a production from initial concept to finished render, including all the steps and keeping files all accessible and readable by Blender.
It's about to get ANOTHER overhaul, integrating it further with Blender's other features such as materials, geometry nodes, etc. In the meantime, if you're at all interested in learning how to use some of its features, I created a FREE course here called DRAW! which should give you a comprehensive rundown of how to access, use, and even illustrate using the Grease Pencil as it currently is.
https://cgcookie.com/courses/grease-pencil-basics-an-introduction-to-blender-s-grease-pencil