I see this phrase used in many essays where it doesn’t make sense.

Bad Example 1: In the world, there are many people who think pizza is delicious.
Where else would the people be? We don’t care about whether people in space like pizza or not!

Bad Example 2: Right now, there are many problems in the world.
There might be problems on Mars, but there are probably more of them on Earth. This is obvious!

When you write, avoid using “in the world” to refer to a location, unless you want it to be clear that you do not refer to space.

Good Example 1: Your English is the best in the world!
Yes! Your English is excellent – not only in your school, town, or country, but in every country. Here, “in the world” gives us a limitation that emphasizes our idea.

Good Example 2: In the world, people have to worry about gravity, but in space, everything is weightless.

When you write, be clear that you are using “in the world” to show the limits, not the location.