Question

What is the need for pre assigned udp port numbers?

Answer 1

I think this question comes under Computer Science.
Pre assigned UDP ports are required for some system tasks. System needs ports to perform several basic functions.

Answer 2

A certified port is a network port which is allocated by the Internet Assigned Numbers Authority (IANA)  for application with a specific protocol or purpose.

Ports with figures 0–1023 are described as the system or well-known ports;

ports with figures 1024-49151 are described as the user or registered ports,

and ports with figures 49152-65535 are described as dynamic or private ports.

Both system and user ports are utilized by transport protocols (TCP, UDP, DCCP, SCTP) to symbolize an application or assistance.