What does "on grid" do in TikZ?

On page 60/1318 TikZ manual there is an example to draw a Petri net as below:

Below is my code:

documentclass{standalone}
usepackage{tikz}
usetikzlibrary {arrows.meta,backgrounds,fit,positioning,petri}
begin{document}

begin{tikzpicture}[node distance=1.3cm, on grid,>={Stealth[round]},auto,
every place/.style= {minimum size=6mm,thick,draw=blue!75,fill=blue!20},
every transition/.style={thick,draw=black!75,fill=black!20},
red place/.style= {place,draw=red!75,fill=red!20},
bend angle=45, every label/.style={red},
place/.style={circle,draw=blue!50,fill=blue!20,thick,
inner sep=0pt,minimum size=10 pt},
transition/.style={rectangle,draw=black!50,fill=black!20,thick,
inner sep=0pt,minimum size=10 pt}, bend angle=45,
pre/.style={<-,shorten <=1pt,>={Stealth[round]},semithick},
post/.style={->,shorten >=1pt,>={Stealth[round]},semithick}]

node [place, tokens=1] (w1) {};
node [place] (c1) [below=of w1] {};
node [place] (s) [below=of c1,label=above:$sle 3$] {};
node [place] (c2) [below=of s] {};
node [place,tokens=1] (w2) [below=of c2] {};
node [transition] (e1) [left=of c1] {}
edge [pre,bend left] (w1)
edge [post,bend right] (s)
edge [post] (c1);
node [transition] (e2) [left=of c2] {}
edge [pre,bend right] (w2)
edge [post,bend left] (s)
edge [post] (c2);
node [transition] (l1) [right=of c1] {}
edge [pre] (c1)
edge [pre,bend left] (s)
edge [post,bend right] node[swap] {2} (w1);
node [transition] (l2) [right=of c2] {}
edge [pre] (c2)
edge [pre,bend right] (s)
edge [post,bend left] node {2} (w2);
end{tikzpicture}
end{document}

I could not understand what "on grid" does. Any idea?