It has no special meaning in a list or dictionary, but can be useful when using source code change management tools, see below.
Non-empty tuples are defined by using a comma between elements, the parentheses are optional and only required in contexts where the comma could have a different meaning.
Because the comma defines the tuple, you need at least one comma if there is just the one element:
>>> 1
1
>>> 1,
(1,)
>>> type((1,)) # need parens to distinguish comma from the argument separator
<type 'tuple'>
The empty tuple is defined by using empty parentheses:
>>> type(())
<type 'tuple'>
The trailing comma can be helpful in minimising how many lines changed when adding new lines; adding an additional line to a dictionary with a trailing comma would not change the last existing entry:
a_value = {
key1: value1,
key2: value2,
# inserting here doesn't require adding a comma
# to the preceding line.
}