The purpose of Wordpress Code Reference is that is an online resource that is auto-generated to help developers find information on Wordpress functions, classes, hooks and more. All code reference content is generated with an open source called WP Parser and the program will parse through all of wordpress's core files then generates reference entries from wordpress's core inline documentation. A search option is available when searching for a code reference that can help you find a certain function if you are not aware of the name. You are able to find various topics such as functions, hooks and methods.
The purpose of the date() function is to retrieve and display the date of a post that was currently written. It will only output the date if the new post's date is different from the last one. A date listing will show only once per day of posts shown in a loop, even if the function is called multiple times for each post.
$format Parameter is the PHP date format and will default to 'date_format' option. $before parameter will output before the date with default being ''. $after will output after the date with the default also being ''. $display will give you the option to either echo the date or return it with default being "true". Then "return" allows you to retrieve the strings from the format parameters.
In conclusion, Wordpress code reference is a useful tool book to help developers with debugging wordpress, function reference, global variables, plugin API's, and shortcode API's. The date() function is an important function to know when dealing with blogs and other subjects that might need the use of a date such as Youtube for example, they always show the date of when the video was posted.