Since tuples are immutable, they can be dictionary keys:
# Mapping coordinates to values
grid = {}
grid[(0, 0)] = "origin"
grid[(1, 2)] = "point A"
print(grid[(0, 0)]) # "origin"
This is useful for grids, graphs, and any situation where you need compound keys. You can't do this with lists:
grid[[0, 0]] = "origin" # TypeError: unhashable type: 'list'
If you need a key made of multiple values, use a tuple.