Snowden is a whistle blower for US surveillance. To some he is a hero, including myself. But to others he is a bad guy who shouldn't have revealed state secrets. Anyone interested should make sure they have watched the Will Smith movie, Enemy of the State. Great movie. Even though it is fictional it shed light on something we knew was true and growing.
As the world changes (and I'll try to make this less of a book) the face of war and economics change. The same way companies want your information to sell you products, your data is valuable in other respects. The age of information is the age of intelligence. Imagine a war where none of the fighters can communicate and where they're all dressed the same. Threats in the modern era aren't so much from other governments and powers who desire conquest. There is still some of that but its mostly cyber threats, countries competing like companies.
Do governments need to spy on us? Technically... yes. In this age? Yes.
Reason being, threats use communications and guerrilla tactics to infiltrate. So you have to tap into those communications and spot their allies and potential allies because certain things you just cannot do or buy or obtain without the right documents, licenses, etc. So in order to keep your own people safe from the people who are walking past them on the street everyday, whether they are plotting to build bombs or just plotting to mass murder people from some hotel room in Vegas... the only way you can get ahead of these things before there's a mushroom cloud or bodies dropping is to use some measure of surveillance. No one likes it but most people would rather walk through an xray than to wonder if the plane they're getting on could be hijacked.
So even though Snowden would agree that some measure of this is necessary for our security, what he saw went too far in his opinion. The capabilities you need to have may need to be completely insane FOR WHEN YOU DO NEED IT. But having a weapon in your arsenal means you shouldn't pull it out unless you need to. And you have to make it so that weapon, paid for and developed by the people, isn't manipulated or exploited by corrupt people meaning to do them harm.
So Snowden rang the alarm to tell Americans about the capabilities and potential of the government. Since these systems can be thought of as still being relatively young, it's like having a child that you know is going to be 7' tall and 300lbs but not teaching him any morality. You have to have balance. And even in the secular world of government that means you must have ethics and rules and regulations.
So his fears aren't just about where we are but where we go from here. Eventually, an AI will most likely be spotting you on camera and checking against a database of behaviors for anything suspicious and it will notify nearby officers to try and prevent crime. I can't know this for sure because it partially depends on how we vote and what we allow in the future. And AI might tag every word you've ever said online back to your real identity and you could be on watch lists because of statements you made being similar to some one else's ideas who might be a violent extremist. Snowden wanted us to start thinking about this; to have conversations about this, not simply sit here while it develops behind and watching our backs.