Hello Gagan,

We need to classify drugs in different ways because different types of classification to drugs may be helpful for people belonging to various fields, such as scientists, drug manufactures, students, chemists, etc.
Thus, the knowledge of the pharmacological effect makes the job of doctors easy. Also, correct medicine can be given as first aid in case of emergency.
For example- When there is acidity, we can use antacid.

This classification provides doctors with the whole range of drugs available for the treatment of a certain type of problem

The action of drugs on a particular biochemical process

This classification provides a range of drugs sharing common structural features

Drugs have some common structural features may have the same mechanism of action on the targets. Hence it is the most useful classification for medicinal chemists.

Good luck,

Hope it helps :)