An idea is expressed twice in different words. The use of varying words helps define ideas that some might not understand. It also conveys the exact thought since words in a language carry a range of implied meaning.
The heavens declare the glory of God;
And the firmament shows His handiwork.
Day unto day utters speech,
And night unto night reveals knowledge.
Sometimes there are double matches, one within a pair of lines and other between two pairs of lines.
The earth is the LORD'S, and all its fullness,
The world and those who dwell therein.
For He has founded it upon the seas,
And established it upon the waters.
Not every element has to match. In doing so, the poet shows that the same characteristics match different groups.
There the wicked cease from troubling,
And there the weary are at rest.
The synonyms can also appear in the same line.
But I am a worm, and no man;
A reproach of men, and despised by the people.