Hello shialaja, to explain a little about why to use design patterns i will use a metaphor or analogy, i will translate you to the building construction on an architecture point of view.
Why do everyone uses columns when building an structure?
- It's not only because it's been used since ages ago, but also because it solve, address and specific problem. There nothing better to hold and create and maintain and structure steady and it can support a lot of weigh.
Why do we use windows?
and so on,
You'll see that these things that we see on the buildings or the houses follows some patterns that solves an specific problem. When an architect sees some kind of behavior or wants to achieve something, the first thing that comes to it's mind it to look for something similar or someone that done the same before, because easier to understand and it's part of some kind of standard. If that prototype that he found solve perfectly the architect need then most of the work it's already done or if not solve the problem completely it's just only to change a few things but the main idea will be the same.
Since the beginning of software design and software development the architects have found several prototype or some desired behaviors and have seen some kind of pattern in then and they have documented the solutions that they use to solve a series of specific problems.
That why the design pattern exists and are useful for,