Why do we use the word "thereof"?

Firstly I am not very good in English, so pardon me, is my question sounds too silly.

• Why we use "thereof"?

It is a formal way of referring to something just mentioned.

formal

of the thing just mentioned;
of that: the member state or a part thereof.

I like this example: "This chair is suited to your life or lack thereof."

Here thereof refers to the life you may or may not have, and the chair will suit you in either case.

It reflects the fact that its is a relatively modern word. Until about 1550, his was used for both modern his and modern its. When that use of his became obsolete, it was replaced by thereof. On the Liberty Bell, this verse from the King James Bible appears: "Proclaim liberty throughout all the land unto all the inhabitants thereof." In modern language we would say "all its inhabitants", but the word its does not appear in original editions of the King James Bible. Because of this use of thereof, it survived until modern times in highly formal, legal, and religious language and parodies of them ("parodies thereof", I might say).

Here's another example usage of the word:

Instead of:

The truth of the matter will be determined by the quality and quantity of the evidence.

..The writer may opt for:

The truth of the matter will be determined by the evidence (the quality and quantity thereof).

..But I agree with the other folks that the word can be replaced by "of it":

The truth of the matter will be determined by the evidence (the quality and quantity of it).