It’s not open-source, so practically the question is equivalent to “why reinvent the wheel by creating libreoffice when there’s a perfectly good Microsoft office suite out there”
In case anyone gets confused, PDFTk Server is just the name for the CLI tool, which hasn't been updated in 10y+ (and annoying to compile due to newer GCC versions removing GCJ). The pdftk provided in various distros (incl. Debian, Fedora, Arch, NixOS) is pdftk-java, a 3rd-party pure Java port of the original tool.
This was my first thought, but after reading the comments here, I see I had no idea how many other alternatives already existed, so why not add another one.
Not sure if this particular library is an improvement, but even if it serves nothing but the author’s enjoyment, or education, it’s a win.