Head section benefit: they get to run before the rest of the HTML content is processed. Realistically though there is no need for this now.
Head section downside: they delay the loading of the page, and result in a slower user experience.
Body section benefit: the HTML is already loaded so scripts can easily interact with the page, and people get to see the page load much sooner
Body section downside: Some positively ancient browsers that don't understand HTML 4 can only load scripts from the head, such as IE4 and Netscape 4
Because the body section benefits are so great and the downside so minimal, loading scripts from the end of the body has become the preferred standard fro web development today.