There are many reasons why it is important to cite work borrowed from other sources in academic writing.
The simplest of these is that most, if not all, schools require it.
After that, there is the point of honesty.
Third, to demonstrate learning and knowledge of a field. By that I mean, often you are attempting to show that you are familiar with the scholarship in a given area, that you know which sources are most important, and that you can distinguish between ideas developed by different thinkers. Citations show this.
Then, courtesy for your readers. Assume your readers are interested in what you have to say, and will want to do more reading in the subject. You're telling them where to find useful sources.
These don't appy the same way in creative works, by the way; different rules apply.