The Government,secret services and the police. It's used for counter terrorism as well as triangulating criminals calls but stealthy without the bounds,restrictions and red tape caused by other methods we know about but these are inadmissible in courts.
Using this they can observe,monitor and then when 100% they have a suspect/suspects they then ask courts for warrants to use traditional methods which they then can use in courts not mentioning anything about the stingrays which officially don't exist.
If you're worried about sexual text messages or dodgy photos you've sent or received then don't worry as as mi5 and the secret organisations have more to worry about then you asking your wife/mistress or mistresses what they're wearing tonight and what are they going to do to you or want done to them although they probably do have a quick chuckle before moving onto more serious national security issues.
/All this spying stuff does not bother me, I don't intend doing anything overly criminal anytime soon/
It's not always you they target but maybe spying on your family/friends using you as the conduit.
Yes "They" can use your smart tv,as well as your phone,computer and tablet to spy on you and it's not only criminals who do this but advertisers and large so called above board companies who turn out to be shady corporations who listen to you as well.
As stated myriad times before there aren't enough people in the world working full time to read every message sent by 'innocent' people
Once you press 'send' on any message it's out there, potentially to be freely read by anyone unless you choose to draw attention to yourself by using PGP or something of course :-)
We cheerfully set yahoo, Instagram Facebook etc etc store our messages and photos without a second though too
On that note I have to dissipate into society making a careful note not to run into any tube stations wearing a rucksack,poor guy..See you later and remember,if you're transparent and have nothing to hide then you have nothing to fear but if your circle of friends or family do illegal or immoral things then expect to be watched and monitored at some point.
Makes you wonder what that van that has been parked outside for the last 3 weeks outside your house has been doing now doesn't it.