Purpose refers to the intent of the information.
Is the source trying to inform you of something?
Most sources are trying to be informative - newspapers, magazines, scholarly articles, etc. The level of detail - and sometimes the level of accuracy - they provide will often depend on their intended audience. For example, a scholarly article is likely to be more detailed and precise than a short newspaper article.
Is the source trying to convince you of something?
Are they trying to convince you of something that is open to interpretation? For example, is someone trying to argue that a certain law should be repealed?
As we mentioned in our discussion of credibility, scholarly literature often involves continued discussion and debate over a particular topic. Scholars will usually provide evidence to support their position or disprove another's position.
Are they trying to convince you of something that can be disproven? For example, is someone trying to argue that plants do not need light and water to survive? If so, they are probably not a credible source.
Is the source trying to describe their own experience of something?
If so, make sure to identify what parts of that description are factual ("It was hot that day, over 90° F...") and what parts are personal ("I enjoyed the warmth").
Is the source trying to sell you something?
Be sure you can differentiate between a website that is an ad, versus a website that contains ads.
Be critical of any claims and evidence that advertisements present to you. For example, perhaps a skin care company is claiming that research has shown a particular ingredient to be effective. Did they sponsor that research? Are there any neutral parties that can confirm their claims?