Can someone tell me why exactly we use this pic for this project and what are the particuliarties of this pic

 

The datasheet can tell you better than we can...

 

Can someone tell me why exactly we use this pic for this project and what are the particuliarties of this pic

What is the point of reference for this?
I have used the 18f2221 for tachometer purpose and comparing them I don't see an edge in using the 2550?
Max.