The `IFNULL` function in Snowflake is used to replace null values in a column or expression with a specific replacement value. It helps maintain data consistency and accuracy, preventing downstream issues such as skewed results, inconsistent formats, and rejections by downstream systems. The syntax for `IFNULL` is straightforward: `IFNULL(expression, replacement)`. Common reasons to use `IFNULL` include disruption of downstream analytics, creation of inconsistent data, breaking downstream processes, and lowering data quality and usability. Other functions like `NVL`, `COALESCE`, and `IS NULL` can also be used for managing null values in Snowflake, depending on the specific requirements.