Short: You don’t need .html ending.
.html ending can be easily archive by setting in Options > Permalink or using plugin like Custom Permalink.
But that’s just personal style. Having html or not is now just different in length of URL. For me, the shorter one is better. It’s has no effect on SEO score. In the past, .html ending is some kind to distinguish between archive and single/page, but now the Search Engine is smart enough to now the page is archive or single. So why don’t we use shorter URL.