Spaces can visualize where the tokens end.
For example:

Code:

x=1; echo "$(( x + 1 ))"
echo "$( ls )"
echo "$(( $( wc -l </etc/passwd ) + 1 ))"

All echo arguments are in "quotes", where $var and $( ) and $(( )) are evaluated.
But not in 'ticks', for example

Code:

echo '$( ls )'

And see what an unquoted

Code:

echo $( ls )

does!
The "quotes" are mostly used in command arguments; they evaluate $ as shown but not special characters like *.
An assignment like

Code:

x="$(( x + 1 ))"

does not need quotes, because the right side is not a command argument.
For quotes within quotes, one can sometimes choose the different quote types.

Code:

echo 'a part with "double-quote" within single-quotes'
echo "a part with 'single-quote' within double-quotes"

Or use a \ single-character quoting, preferably outside the quoted strings, using a simple string concatenation.

Code:

echo 'a part with '\''single-quotes'\'' withing single-quotes'
echo "a part with "\""double-quotes"\"" withing single-quotes"