I'm not sure if Visual Voicemail really uses silent SMS, but even older phones had a series of indicators such as "voicemail waiting", "message waiting" etc. which the network could control via binary SMS payloads.
By sending one that clears all of them in a network that doesn't use them (or sending one equivalent to the current state for one that does), you can achieve the outcome of initiating SMS-MT (mobile-terminated) delivery to a given ME (phone) without any user notification.
SMS delivery by necessity involves paging the device, revealing its location at a finer level (base station instead of paging area).
So I wouldn't say silent SMS were designed as a spying tool, but they're one out of several ways to silently "ping" a phone and force it to communicate with the network without having to wait for it to cross location area boundaries, get or make a call etc.
By sending one that clears all of them in a network that doesn't use them (or sending one equivalent to the current state for one that does), you can achieve the outcome of initiating SMS-MT (mobile-terminated) delivery to a given ME (phone) without any user notification.
SMS delivery by necessity involves paging the device, revealing its location at a finer level (base station instead of paging area).
So I wouldn't say silent SMS were designed as a spying tool, but they're one out of several ways to silently "ping" a phone and force it to communicate with the network without having to wait for it to cross location area boundaries, get or make a call etc.