Why we need clock pulse in sequential circuits?
First of all, not all sequential circuits (circuits with feedback) have clocks.

However, the design of asynchronous state machines is very esoteric, requiring the consideration of all possible glitch and race paths through the logic in order to get it right, and I suspect that it is not generally taught these days.

Therefore, simple asynchronous state machines are encapsulated in the form of standard types of clocked flip-flops (T, D, S-R, J-K, etc.) and all higher-order circuits are built using clocked techniques, which vastly simplifies their design. It also vastly simplifies the design of software tools that can synthesize such circuits.