There are many ways to explain why you are doing something or what's the use of things. This article will explain another way to do that, and it just happens to involve "是...的". This isn't to say that you can't make these sentences without using "是" and "的" but this can help emphasize why you are doing something or what's the use of thing, giving your sentence a little more kick.

Structure
subject+是+reason/purpose+的

If the subject is a person, there is often a "来" or "去" after the "是", indicating direction like "coming here" or "going there". If the subject is a thing, "用来" is often used. Look at the examples below for some more clarification.

person+是+来+reason/purpose+的
thing+是+用来+reason/purpose+的
Examples
Wǒ shì lái shànɡ hǎi lǚ yóu de.
我 是 来 上海 旅游 的。
I came to Shanghai to travel.