13.32.6 Quotes and Braces

The distinction between braces ({ }) and quotes (" ") is significant when the list contains references to variables. When references are enclosed in quotes, they are substituted with values. However, when references are enclosed in braces, they are not substituted with values. For example, see this below table.
With BracesWith Double Quotes

set b 2

set b 2

set t { 1 $b 3 }

set t " 1 $b 3 "

set s { [ expr $b + $b ] }

set s " [ expr $b + $b ] "

puts stdout $t

puts stdout $t

puts stdout $s

puts stdout $s

will output

1 $b 3           vs.            1 2 3
  [ expr $b + $b ]                      4