Why some people use could instead of can (in the simple present tense) For example,
Could you open the door?
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Could is a modal auxiliary and so is can. The difference is the context it is used in.
Can is the more informal of the two words and is used while addressing friends and siblings (while making a request).
Could is the more formal of the two words. It is used while acknowledging someone elder or a respected personality. It is often used while communicating with strangers since it is more polite than the word can (while making a request).
Apart from the above mentioned, there are other scenarios that are taken into consideration while using the two words.
Could is the past tense of the modal verb can. Hence one must be careful while using it in affirmative or declarative sentences.
For further detailed information, refer to this article.
Hope this helps. Thanks.