In React, a component re-renders whenever there is a change in it's state or one of it's props.
The reason it behaves like this is so that it would be possible to "react" to a change in the mentioned data, and to reflect UI changes accordingly.

Every time the component re-renders, so does any logic within it that is not cached (functions, variables, etc..)

useEffect helps us with reacting to a change in the state or props that are stated in it's dependency array.
It gives us to option to automatically run a callback function in such an event/s.

useEffect with an empty dependency array, will run only a single time when the component is mounted.

So in this example -

function myComponent() {
  // some states, variables

  useEffect(() => {
    // do something on mount <= this is my current concern.
  }, [/* empty */]);

  // return Element ...
}

The callback function inside the useEffect will run only once, when the component is first "brought to life".
Subsequent renders will not invoke this.

While in this example -

function myComponent() {
  // some states, variables

  // do something on mount <= this is my current concern.
  
  // return Element ...
}

Whatever you put in there will run every time the component re-renders.
Whether this is ok or not depends on your use-case and what function are you trying to run, if it's "cheap" to run or not, etc..

They will be executed every time at component re-render if you put them in function directly

As you mentioned, when you have an empty dependency (like below), the code inside will only run on mount.

  useEffect(() => {
    something() // only runs on mount
  }, []);

If you don't have a useEffect at all, the code will be run every time the component rerenders.

function myComponent() {
  // some states, variables

  something() // runs on every rerender
  
  // return ...
}

Now the question is, "when does a rerender happen?". In general, anytime a parent component renders, React will rerender all children of that component. So rerenders can occur quite often.

Look at this article, which has some really helpful visual examples of when components rerender.