why in some circuit there is use of bubble followed by bar?
You're asking why there is a double negative.

Typically in a circuit diagram, the elements will be real, purchaseable or manufactureable things. Many logic chips have active-low signals, such as CL(ea)R, so they come with that "bubble" baked in, indicating their inverted logic.

So, when a designer needs a flip-flop which has that active-low input, they will often label the net (wire) with what the signal means for the application, which could mean it too has negative, active-low logic.