LaTeX: Why causes no paragraph break for fully justified text, but does so for raggedright, raggedleft, and centering?

Note: This question is not about getting a desired visual effect in pdf, it is concerned with LaTeX/TeX internal algorithm.

In the following code, presence of sometimes produces a new internal tex-paragraph when encountered in the middle of a user-paragraph, and sometimes does not. In normal text mode, the only time it seems to not produce a new paragraph is when in fully-justified (default) alignment mode.

So the question is why did LaTeX team code this discrepancy? Was it to avoid changing the paragraph indentation for partial tex-paragraphs? Is it any better than starting a new paragraph with no indent in fully justified mode (like parvspace{1cm}noindent instead of [1cm])?

Code (lualatex):

% >> lualatex par_vs_backslashes.tex
documentclass{article}

directlua{
    nodetree = require"nodetree"
    function filterprint(h)
        nodetree.print(h)
        return h
    end
    luatexbase.add_to_callback("pre_linebreak_filter",filterprint,"filterprint")
}

begin{document}

% This produces 2 tex-internal paragraphs
Hello[1cm] fooparvspace{1cm} world

raggedright

% This produces 3 tex-internal paragraphs
Hello[1cm] fooparvspace{1cm} world

end{document}

Console:

% Output for fully-justified mode:

├─LOCAL_PAR 
├─HLIST subtype: indent, width: 15pt
├─GLYPH subtype: 256, char: H, width: 7.5pt, height: 6.83pt
├─GLYPH subtype: 256, char: e, width: 4.44pt, height: 4.48pt, depth: 0.11pt
├─GLYPH subtype: 256, char: l, width: 2.78pt, height: 6.94pt
├─GLYPH subtype: 256, char: l, width: 2.78pt, height: 6.94pt
├─GLYPH subtype: 256, char: o, width: 5pt, height: 4.48pt, depth: 0.11pt
├─ADJUST 
│ ╚═head:
│   └─GLUE width: 28.45pt
├─PENALTY penalty: 10000
├─GLUE stretch: +1fil
├─PENALTY penalty: -10000
├─GLUE subtype: spaceskip, width: 3.33pt, stretch: 1.66pt, shrink: 1.11pt
├─GLYPH subtype: 256, char: f, width: 3.06pt, height: 7.05pt
├─GLYPH subtype: 256, char: o, width: 5pt, height: 4.48pt, depth: 0.11pt
├─KERN kern: 0.28pt
├─GLYPH subtype: 256, char: o, width: 5pt, height: 4.48pt, depth: 0.11pt
│ ╚═  properties: {['injections'] = {['leftkern'] = 18350.08}}
├─PENALTY subtype: linepenalty, penalty: 10000
└─GLUE subtype: parfillskip, stretch: +1fil

├─LOCAL_PAR 
├─HLIST subtype: indent, width: 15pt
├─GLYPH subtype: 256, char: w, width: 7.22pt, height: 4.31pt, depth: 0.11pt
├─KERN kern: -0.28pt
├─GLYPH subtype: 256, char: o, width: 5pt, height: 4.48pt, depth: 0.11pt
│ ╚═  properties: {['injections'] = {['leftkern'] = -18350.08}}
├─GLYPH subtype: 256, char: r, width: 3.92pt, height: 4.42pt
├─GLYPH subtype: 256, char: l, width: 2.78pt, height: 6.94pt
├─GLYPH subtype: 256, char: d, width: 5.56pt, height: 6.94pt, depth: 0.11pt
├─PENALTY subtype: linepenalty, penalty: 10000
└─GLUE subtype: parfillskip, stretch: +1fil

% Output for rest of the modes (raggedright, etc):

├─LOCAL_PAR 
├─HLIST subtype: indent
├─GLYPH subtype: 256, char: H, width: 7.5pt, height: 6.83pt
├─GLYPH subtype: 256, char: e, width: 4.44pt, height: 4.48pt, depth: 0.11pt
├─GLYPH subtype: 256, char: l, width: 2.78pt, height: 6.94pt
├─GLYPH subtype: 256, char: l, width: 2.78pt, height: 6.94pt
├─GLYPH subtype: 256, char: o, width: 5pt, height: 4.48pt, depth: 0.11pt
├─PENALTY subtype: linepenalty, penalty: 10000
└─GLUE subtype: parfillskip, stretch: +1fil

├─LOCAL_PAR 
├─HLIST subtype: indent
├─GLYPH subtype: 256, char: f, width: 3.06pt, height: 7.05pt
├─GLYPH subtype: 256, char: o, width: 5pt, height: 4.48pt, depth: 0.11pt
├─KERN kern: 0.28pt
├─GLYPH subtype: 256, char: o, width: 5pt, height: 4.48pt, depth: 0.11pt
│ ╚═  properties: {['injections'] = {['leftkern'] = 18350.08}}
├─PENALTY subtype: linepenalty, penalty: 10000
└─GLUE subtype: parfillskip, stretch: +1fil

├─LOCAL_PAR 
├─HLIST subtype: indent
├─GLYPH subtype: 256, char: w, width: 7.22pt, height: 4.31pt, depth: 0.11pt
├─KERN kern: -0.28pt
├─GLYPH subtype: 256, char: o, width: 5pt, height: 4.48pt, depth: 0.11pt
│ ╚═  properties: {['injections'] = {['leftkern'] = -18350.08}}
├─GLYPH subtype: 256, char: r, width: 3.92pt, height: 4.42pt
├─GLYPH subtype: 256, char: l, width: 2.78pt, height: 6.94pt
├─GLYPH subtype: 256, char: d, width: 5.56pt, height: 6.94pt, depth: 0.11pt
├─PENALTY subtype: linepenalty, penalty: 10000
└─GLUE subtype: parfillskip, stretch: +1fil

Output:

documentclass{article}

parskip=1in
begin{document}

begin{center}
  aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
  aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
  zzzlinebreak zzz aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
  aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
end{center}

begin{center}
  aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
  aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
  zzz zzz aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
  aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
end{center}

begin{center}
  aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
  aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
  zzzpar zzz aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
  aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa aaaaa
end{center}

end{document}