Why does Vim add spaces when joining lines?

I have a feeling this is what you really want: gJ

From :h gJ:

gJ          Join [count] lines, with a minimum of two lines.
            Don't insert or remove any spaces.  {not in Vi}

This is handy if you’ve copied something from a terminal and it’s pasted it as a big rectangular block into vim, rather than a single line.

I usually use it in visual mode. Hilight stuff, gJ.