What is defaultdict(set) in Python

I was solving leetcode and had to use defaultdict(set) for a problem. may you explain please, What defaultdict with set argument will create that normal dictionary can not ? also what is the use of set as a argument and why I used it here.

I was searching over internet about this but couldn't manage to find a precise answer. Here is the code where I used defaultdict(set),

class Solution:

def isValidSudoku(self, board: List[List[str]]) -> bool:
    rows = collections.defaultdict(set)
    cols = collections.defaultdict(set)
    square = collections.defaultdict(set)

for r in range(9):
    for c in range(9):
        if board[r][c] == ".":
            continue
        if (board[r][c] in rows[r] or board[r][c] in cols[c] or board[r][c] in square[(r//3, c//3)]):
            return False
        
        rows[r].add(board[r][c])
        cols[c].add(board[r][c])
        square[(r//3, c//3)].add(board[r][c])
        
return True