Merging can be thought of as a complementary process to the selection that we read last time.
One of the most impressive property of merge sort is that it can sort a file in $N\log N$ , independent of input. ( Heapsort also does this)
Prime disadvantage of mergesort is that it requires extra space proportional to $N$.
A guaranteed $N\log N$ running time can be a liability. For e.g. we know we can sort special cases in linear time in some other sorting schemes. That is it become significant when there is already a order in the file.
Mergesort is stable sort. Quicksort and Heapsort are not stable.
Mergesort is a method of choice for sorting Linked Lists ( why ?) {
Linked list is a linear structure and merge sort access the structure in a linear fashion.
}