because they decrease the time complexity of the program we 
have to do.

By using the linked list we can allocate definite memory
space  for variables.But in array cannot allocate definite
memory space.

In Linked list we can insert or delete the elements at any 
position,which is not possible in arrays and arrays are of 
fixed size,but linked lists are not have a definite length. 
we can dynamically allocate the size of linked list with 
out wastage of memory...

we can decrease our time complexcity and allocate 
[particular memory place for give variable,this is not 
possible in the arrays.this is my ans.......

Linked list has an overhead cost of maintaining reference
pointers it uses. They too consume lot of memory. And this
cost is more in case of doubly linked lists. 

Linked lists always provide sequential access. While arrays
give random access.

Use of array: All the elements are lumped together in one 
block of memory. Individual elements can be accessed thru 
Array index. Random access & Dynamic allocation are 
possible.

Use of Linked list: Linked list allocates space for each 
element separately in its own block of memory called 
a "linked list element" or "node". It gets the overall 
structure by using pointers to connect all its nodes 
together like the links in a chain. Here the elements of 
list can be accessed thru memory address (Since the pointer 
stores a reference to another variable)which improves the 
efficiency. Size of the list can also be modified (ie) we 
can insert/delete elements of the list.

Linked List and array both are used continuous allocation of
memory. When we want to assign too many same type of data
sequentially into memory, at that time we use the Linked
list and array because taking separate variable for each
data is tedious and time consuming so we use the Linked list
and arrays.

it is not possible to declare variables for each and every
data items in the memory it takes too much time and
increases the size of code and it will be cumbersome to use
these variables