What is the main reason to go with ashx handler files in asp.net ? Means in which type of situations we should think about handlers as best choice ?

Basically, .ashx or handler files are used when you want to do multiple task at a time and that will work in the asynchronous manner. For Example:- When you are uploading large number of files simultaneously from multiple location into the same site then at that time we should use .ashx or handler files.

For independent work. Upload files to certain network drive from temporary web server location.