When should one use a XML sitemap?

Well, i can use XML sitemap on ODK Application to do questioniar digital through smart phone

Sitemaps are a protocol that allows the webmaster for a website to inform Google and other major search engines about URLS on a website that are available for crawling. Sitemaps allow search engines to find all of your webpages, that they might otherwise miss when indexing. The XML sitemap allows you to specify additional information about each URL such as:

 

When it was last updated

How often the site changes

How important the page is in relation to other pages on the site

Having this information within one document helps search engines understand your website and crawl it more intelligently. Sitemaps are an inclusion protocol, where Robots.txt files are exclusionary.

 

There is no guarantee that an XML Sitemap will get your pages crawled and indexed by search engines but having one certainly increases your probability

 

A sitemap is important for the SEO , it's used by google to know the pages it should crawl

I agree with Furqan Muhammad' s answer. The answer for 'When' is as soon as the website is complete and hosted it is good to have a sitemap.xml. Also it should be updated regularly with new urls and its additional details. Now a days platforms like wordpress, magento etc. have plugins and inbuilt sitemap generators which automatically updates sitemap as soon as a new page is added to the site.

I couldn't agree more on the precise answer that Bernard had and to add, its not only Google but other search engine bot and crawlers as well.

XML Sitemap is used to demonstrate the sitemap consistently on website, mobile devices, workstations, etc.

sitemap tell crawlers about site structure and which page should crawl.

use it in ODK applications

because it allows to inform Search Engines about important pages on your website. That increases its visibility to Google, Bing, Yahoo, Baidu, Yandex etc, and ensures indexing of the web pages that might not be discovered otherwise.

It seems that there is a massive debate on whether or not XML sitemaps should be used. My understanding so far is that SEO-wise it's links that matters and that XML sitemaps won't neccesarily improve your ranking but will just ensure that your sites are indexed more quickly. So I would like to know when to use a XML sitemap and what the potential pitfalls are.

XML sitemap is a way for website owners to tell search engines about all the pages that exist on their website.