Let's assume that this question is referring to every digital calendar ever made. As in, even those made on devices without internet connectivity or any other fancy features that we utilize with many digital calendars today.
If a planning device that came with a digital calendar didn't have internet connectivity, it wouldn't be able to be connected to the cloud. Back when these devices were around, it wasn't even plausible to store things like calendar events and contacts in what we know as the "cloud". Also, these devices probably had to be hardwired to a computer and new software had to be downloaded to them, so the updates weren't automatic. Archaic, right?
Also, consider the fact that even reputable websites/companies such as Google with Google Calendars or Apple with iCalendar will never go without their malfunctions or threats. Sometimes, these websites can be hacked and the data that they contain can be compromised, especially if they're stored on the cloud. Also, even though it's not realistic, Google or Apple could one day decide to completely get rid of their calendar programs altogether. So, this means that digital calendars are definitely able to be destroyed or lost.
That just leaves "They can be used anywhere". This is true, even with those archaic devices specifically used for scheduling events and such. Nowadays, you have your phone and possibly a laptop on you at all times, which will likely have a calendar on it as a stock application. So, I think this is your answer.
