Sitemaps are not essential but count towards good user experience so in general you should have one (as a part of your site).
Google sitemaps is different though – in my experience there is no hard evidence that submitting sitemap to Google helps in indexation or rankings. In past there were many reporting their indexed pages count dropping as a direct result of submitting the sitemap although I believe most issues are solved by now. Also, you are giving Google a lot of information about your sites; not always a good idea especially if you are keen on conspiracy theories.
I submit them personally, but not for each and every site. The data you can get from them how Google handles your site(s) also comes handy sometimes.
If you are thinking about submitting one to Google sitemaps, do this:
– google “sitemap generator”, 1st result is quite a good one
– put the created sitemap in the folder with your website files
– validate the sitemap with Google; in Google sitemaps account you`ll get a file which you must put in the same folder as in previous step
Don`t forget to recreate the sitemap as you add new content :wink-wink
cheers