JSON does a couple things really well, and most other things terribly.
But the things it does well are pretty valuable. So in the "strengths" category I'd put the two following points:
1. JSON is very easy to read and understand as a human
2. JSON stuck to the basics. No comments, no references, no clever tricks, and not much space to let folks try to hammer in cleverness (see - no comments).
Neither of those are all that much related to JSON itself as a format - the semantics are basically an accident of timing around JS syntax from the 2000s.
But it's very, very useful to be able to get the raw text for a network message and know exactly what's getting sent without having to have a whole specialized tool framework to parse and understand the message.
It's also useful to not let the spec get so complex that I never want to do that, even if I could (see: xml).
So with JSON - I can easily read the actual network request and understand it, even with essentially zero additional tooling AND I have a very good chance of literally being able to open a text editor and create a new message with valid syntax without any other tools or references.
Further - this holds true even if I'm not an industry expert with 20 years of experience. Most random people off the street can do it with only a couple minutes of coaching.
Not many other serialization formats can do that.
Imagine taking your 8 year old, sitting them down in front of the computer and legitimately saying "JSON doesn't know what an integer is in the spec"!
It's true... but it's absolutely not the point. For normal people "number" is complex enough. And if you need an int and not a float... you can do that processing just fine after getting a JSON payload if you'd like. It won't be as fast as a specialized format (ex protobuffs), or as flexible (ex XML) as other formats - but that's a far distant concern to "Can I hold the hammer".
JSON is really easy. "Easy" as a strength is wildly discounted, but man is it a winner when you get it. I also think it's surprisingly hard to do.