A timestamp is just a number representing how many seconds (or milliseconds) have passed since January 1, 1970. That's called the Unix epoch, and it's how computers think about time.
Why Should You Care?
Timestamps are everywhere in computing. Database records have them. Log files have them. API responses have them. Being able to convert between timestamps and readable dates helps you make sense of all this data.
I use timestamp converters constantly when debugging, analyzing logs, or just figuring out when something happened.