what is the need of sequence generator being used in digital electronics and where do we find its application

 

I'm not sure do you mean as the CD4017, and they used in the advertisement lights as running horses.

 

Practically every item/component/circuit in electronics has practical applications otherwise they wouldn't exist.
We've had this question posted recently: "What are some examples of applications of a capacitor?"

Someone could ask the same question of a diode, transistor, gate, flip-flop, register, counter, multiplexer, decoder, etc.
Understandably, one has to learn about basic components and circuits before moving on to specific applications. In time, you will get to see the big picture.

What is a sequence generator? A sequence generator is a controller that defines specific tasks to be performed in sequence.
Think of the various functions and tasks that must be performed in a laundry washing machine - cold/hot water solenoids, drain valve, agitate, rinse, spin etc.

In a digital computer, instructions are acted upon in a sequence of fetch, decode, execute cycles. This is controlled using a sequence generator.

Those are just two examples of how a sequence generator could be applied.