To help you avoid using the same word too repetitively, redundantly, recurrently, incessantly, etc., etc.
Why synonyms can be useful
Your writing can sound boring if you continually keep repeating the same words. When you create sentences, you can make them more interesting by using words that mean the same as the word you are speaking about. This allows you to add flavor to your writing.
In order to make language a lot more expressive and interesting you should try to vary the words you use as often as you can.
Synonyms for (noun) undersurface
Synonyms:underside, undersurface, bottomDefinition: the lower side of anything
Hypernyms:side, faceDefinition: a surface forming part of the outside of an object Usage: he examined all sides of the crystal; dew dripped from the face of the leaf
Definition: (adj) marked by precise accordance with details Synonyms: meticulous
Usage: meticulous research; punctilious in his attention to rules of etiquette