Note that this was an exaggeration of my experience with LaTeX, it's not like I use these commands everywhere. I think its better to let LaTeX do its job. Nevertheless, looseness=-1 can help to cram a few words in a new line into the previous paragraph, which subjectively looks better sometimes and frees up some space. Negative vspace around figures or tables can also be used to make more space for text and avoid unwanted page breaks. Manual linebreaks can come in handy if you switch TeX engines (e.g. pdfTeX -> LuaTeX) and somehow things don't look like they are supposed to look. You can do it right or you can add manual linebreaks here and there to get the same results.