Choose a global constant to set a value to null or an empty string. For example, choose $GlobalConstant.Null.

Two global constants are

1. $GlobalConstant.Null
2. $GlobalConstant.EmptyString

Scenarios for Process Builders:

1. While using update action in Process builder for Contact records and then if we want to clear Account field, then we can use $GlobalConstant.Null. We cannot use $GlobalConstant.EmptyString since the lookup field cannot be mapped to blank string.

2. While using insert or update action in Process builder and if we would like to update any field with blank value, then we can use $GlobalConstant.EmptyString.

Difference between IsBlank and IsNull – https://www.infallibletechie.com/2012/09/difference-between-isblank-and-isnull.html