The short answer to your question is: There are no legitimate uses for decking. Just about everybody who's not a tech-hating luddite uses the matrix for daily activity. Some access it in full sensory virtual reality, others via augmented reality overlays, but none of them are decking.
The precise game mechanics and the in-world technology description has changed over the years and editions, but the key factor is not all virtual reality Matrix access is decking.
What is virtual reality?
Using a virtual reality interface in Shadowrun is not like the modern definition of virtual reality. There's no walking around, there are no clumsy handheld interfaces, there are no heavy goggles - there's no physical interaction at all. Cyberterminals include a component whose entire purpose is to stop your meatsack from reacting to the things that happen in the Matrix.
Shadowrun's version is a direct neural interface, interfacing your brain with the data. While some systems have a programmed metaphor to make it look and feel like you're walking around or reading books, or searching a library, that's just sculpting (and matrix sculpters make big money). In virtual reality, you think what you want and your cyberterminal does the work - assuming you have the access privileges and software rights to do it.
What is a cyberdeck?
I think the distinction is best made with how things existed in the first fifteen years of the setting - the early 2050s through the mid-2060s. In that time (representing the first three editions of the game system), people used a cyberterminal to access the matrix. There were two essential components used to "create" a person's virtual avatar, their persona, a Bod chip and a Sensor chip. The first managed the virtual-physical aspects of the avatar, and the second managed the computer-to-human translation, how well the avatar could "see". There were other hardware and software components involved, too, but for the purposes of the question, these are the important ones.
Security counter-deckers, known as spiders in later years, had an additional component - an Evasion chip. A civilian-legal cyberterminal didn't even have a socket for the chip. It allowed them a defense against invasive code used by deckers - they could dodge, to use the closest meatspace analogy. Legal, civilian matrix users lacked this particular piece of hardware, and were wide open to attack by hostile deckers.
The real magic, the feature that turned a device from a cyberterminal into a cyberdeck was the fourth and final persona chip - Masking. The masking chip allowed the user to actually hide his activity from the spiders. Without it, a person attempting something illicit was little more than a thug breaking into a convenience store in full view of the cameras while leaving a business card behind. With a good masking chip, he was the virtual equivalent of James Bond (minus the explosions).