Why we design algorithm?

Answer 1

The sense in designing Algorithm lies in the frequent use of a certain manipulation of elements.
You don't want to sort everything by your own sorting method when there are much faster codes snippets available. Additionally you can be sure that they deliver always the right result based on the input. The correctness and the complexity of them is tested.
Also they can apply on a bunch of elements. Mergeseort for example can be applied to thousands of data structures. The same is for Insertion Sort