Settings to set to render with cycles

My scene in Render Preview mode:

With Cycles:

With Eevee:

With Cycles with AO checkbox ticked:

My Question is,
Should I always make AO Checkbox ticked to render with Cycles? or Lighting in my scene is dull?

What I understood is that checkbox sets the below flag


In Cycles Standalone samples, use_ao is never set to true but how is it able to render good?

Please help.

Thanks in advance?