As usual with most discussion regarding sitemap, you will see 99% of people recommending them provide not concrete, fact based reason as to any actual benefit it provides.

• It provides no SEO advantage

• The reason that it's useful to users is like saying my primary navigation system and search function is too difficult to use.

• If you have a large website with lots of deep, frequently changing content, Google will recognize this and increase the crawl frequency and depth.

Sitemaps in my opinion or for the "tin foil hat" crowd.

But.. conversely, they don't hurt either.  If you have a plugin or script that automatically runs over and over to update it for you, then one could legitimately say "Why not?"

There are some obscure reasons one might use - like adding a gazillion new pages to an otherwise small website and wanting to get them indexed sooner or later.  But that's really the only one I can think of.