As I mentioned, it makes the mailing process much more efficient.
PDFiller allows you to create and mail PDFs easily, saving you the hassle of getting an envelope, purchasing a stamp, maybe having to go to the post office, etc. For me that has been its greatest strength. The PDF customization interface isn't bad, but it's not immediately usable. It takes a little bit of time to learn how to use it effectively.
Right off the bat: I would not recommend this software to someone who isn't tech savvy or doesn't want a bit of a learning curve, for example, employers I've had in their 50s, 60s, and 70s who don't really "get" technology. The interface is fine if you just want to sign a document, but is not particularly intuitive when it comes to doing simple things like outright deleting portions of text. Too many steps.