You have a large website:
A website with 100 pages isn’t large. Even a website with 1,000 pages is still pretty small, and a sitemap isn’t really necessary. For bigger sites, however, you should definitely consider it. Do you have one million pages? Then you need a sitemap.
You have a new site:
A sitemap helps crawlers to find a brand new website just after launch and to index the new pages much faster.
You change your website’s content frequently
You need to index your fresh content very fast:
A sitemap helps to accelerate the indexation of newly added pages after adding them to the sitemap, especially, when your site is in Google News.
You'll also want to consider what type of community it is, and if indexing will be a benefit. A good example would be a programming forum you might want each thread to show up via google results or in the very least a category... If it were a roleplaying forum, maybe not so much.
Sitemaps are important for SEO because they make it easier for Google to find your site's pages this is important because Google ranks web PAGES, not just websites There is no downside of having an Sitemap and having one can improve your SEO, a sitemap on our website and we include a short description as part of each section to inform the user as to what the page is about We also use our sitemap for people who find a page that no longer exists on the website
I wish there was a JSON sitemap format, as XML is a bit on the bulky side. It helps search engines to find pages and to avoid hitting pages too much (or to assume it isn't updated when it has been and to not crawl it).