why should scientist conduct research before doing an experiment?

Answer 1

To save repeating work which is already done. It makes sense to tread new ground, rather than just do work which has already been done. You can also see if you can improve on earlier work. As Newton said "If I have seen further, it is because I have stood on the shoulders of giants."

By complementing others' work, you advance science.

Answer 2

It helps you predict what could happen. If your experiment involves water it would make things much easier if you know about water. It also helps you understand the outcome of your experiments, thus saving time and making sound theories.