The original purpose of silent SMS was to send voicemail or missed call notifications to handsets, which would trigger an icon to be displayed on the device. Sending a regular SMS would be annoying as the user would have to delete it - after you've listened to your voicemail, another silent SMS can be sent to turn off the notification. Also originally SMS was stored in the SIM itself which had limited memory, so it would be not be very convenient if you didn't receive a voicemail message as your SIM was full. Remember this is a 28 year old feature of GSM.
The tracking argument seems somewhat mute, maybe when this first came to light 10 years ago it wasn't the case, but nowadays I would be very surprised if operators do not keep detailed logs of all the IMEI (unique identifier for a given device) and IMSI (same, but for the SIM) that connect to their towers.
The original purpose of silent SMS was to send voicemail or missed call notifications to handsets, which would trigger an icon to be displayed on the device. Sending a regular SMS would be annoying as the user would have to delete it - after you've listened to your voicemail, another silent SMS can be sent to turn off the notification. Also originally SMS was stored in the SIM itself which had limited memory, so it would be not be very convenient if you didn't receive a voicemail message as your SIM was full. Remember this is a 28 year old feature of GSM.
The tracking argument seems somewhat mute, maybe when this first came to light 10 years ago it wasn't the case, but nowadays I would be very surprised if operators do not keep detailed logs of all the IMEI (unique identifier for a given device) and IMSI (same, but for the SIM) that connect to their towers.