You would use it in the instance when you don't want to call someone. For example, my colleague may be in the datacentre and I'll just ping him a voice "Can you check the lights on server XXXX", he'll say "yep" then come back 2 mins later "all good" or something. Or you could be in the car and say to the Wife "I'm outside the shop", "ok" she can reply. You could do with a text or call, but the walkie talkie is probably quicker and more convenient (if it works lol). Those are just a couple of examples. Not for everyone, but useful.
I remember back in about 2001 when SMS first came about, people saying the same thing - why would you send a text message when you could just call.