It's true that the space between sentences should be larger than the space between words, as it is in properly typeset text, though not twice as large. Using two spaces is a way to approximate that when you only have monospaced fonts. It's no longer encouraged in electronic text when using proportional fonts because the rendering software _should_ adjust the spacing itself.
It "should" adjust, but in practice most software do not render spaces after periods properly. There is no in band signal to distinguish periods for sentence ends vs e.g. Mr. Thus there is a clear argument for double spacing in all contexts
edit: monospaced, not proportional.