[1] Its purpose is to semantically separate characters that should not be considered digraphs as well as to block canonical reordering of combining marks during normalization.
For example, in a Hungarian language context, adjoining letters c and s would normally be considered equivalent to the cs digraph.
However, in contrast to the zero-width joiner and similar characters, the CGJ does not affect whether the two letters are rendered separately or as a ligature or cursively joined—the default behavior for this is determined by the font.
Compare: In the case of several consecutive combining diacritics, an intervening CGJ indicates that they should not be subject to canonical reordering.
[2] In contrast, the "zero-width non-joiner" (at U+200C in the General Punctuation range) prevents two adjacent character from turning into a ligature.