What is EQS context?

Could you please explain me what is EQS context and why should we use it. i have seen many tutos using EQS System without using EQS context.

Contexts are a frame of reference for various tests and Generators. They can be as simple as The Querier or more complex such as All Actors of a Type.

Of note is the fact a Generator, such as the Simple Grid Generator can use a Context that returns multiple locations or Actors. This will create a Simple Grid, of the defined size and density, at the location of each Context.

Contexts can be created in Blueprint and C++.

From EQS Documentation