I’ll share my understanding of a subpoena, but would appreciate hearing from Albaby1 if he reads this thread.

If you issue a subpoena just because you feel like it, it can lead to serious professional, civil, and criminal penalties.

The decision whether to issue a subpoena is typically first with the prosecutor or defense attorneys. The subpoenas exercise the power of the court to compel documents or testimony, but the court generally allows counsel for the sides to determine in the first instance what subpoenas are appropriate. The check on that power is procedural - if a party receiving a subpoena feels the subpoena is unjustified, they can file for a protective order.

Where that breaks down is when the subpoena is issued for records that belong to another party than the one that cares about their privacy. For example, I don't want the government rifling through my phone records to see everyone I've called for no reason. If a subpoena is issued to me asking for my phone records, I can go to a judge and object that there's no basis for the subpoena; but if the subpoena is issued to the phone company, I'll never even know about it. The procedural check that exists to make sure that the subpoena power is exercised appropriately disappears, because for third-party records the person getting the subpoena isn't really the person that cares about whether the subpoena is justified or not.

I haven't been following this issue very closely, though it's easy to see why the government would obtain ordinary people's stuff in a criminal investigation - because many of the targets of criminal investigations have a lot of dealings with ordinary people, and the prosecutors are usually looking at every single thing the target does in order to find out where the wrongdoing (if any) is occuring. If the authorities are tapping a mobster's phone (for example), they'll record not just him talking to other mobsters, but when he orders a pizza or calls his kid's school. A notable foreign intelligence example was Dennis Kucinich, who took a call from a Libyan leader in his congressional office and was chagrined to find out it was recorded - not because they were monitoring him, but because they were monitoring the Libyan.