If someone gets an occasional pdf through email, viewing it that way works perfectly. However, if you happen to use a lot of pdf files in your business, you may have a need to store many of them on a storage card and have access to launch/read them. Can you imagine having a hundred or more pdf's in a hundred or more emails and then trying to find a certain one that you need to see? It could be done, but it would be very time consuming.
I have approximately 450 standards in PDF format that I may need to reference while I'm away from my office. Its too many to run through email or store on the device itself.