Why is this an issue?

When the line immediately after a conditional has neither curly braces nor indentation, the intent of the code is unclear and perhaps not what is executed. Additionally, such code is confusing to maintainers.

if (condition)  // Noncompliant
doTheThing();
doTheOtherThing(); // Was the intent to call this function unconditionally?

It becomes even more confusing and bug-prone if lines get commented out.

if (condition)  // Noncompliant
//  doTheThing();
doTheOtherThing(); // Was the intent to call this function conditionally?

Indentation alone or together with curly braces makes the intent clear.

if (condition)
  doTheThing();
doTheOtherThing(); // Clear intent to call this function unconditionally

// or

if (condition) {
  doTheThing();
}
doTheOtherThing(); // Clear intent to call this function unconditionally

This rule raises an issue if the line controlled by a conditional has the same indentation as the conditional and is not enclosed in curly braces.