Value contains an Object data type. You need to convert the value if you want to store it in another data type. For input parameters, the value is bound to the TAdoDbxCommand that is sent to the server. If the parameter's Direction is Output or ReturnValue, the value is set on completion of the TAdoDbxCommand and after the TAdoDbxDataReader is closed. When sending a null parameter value to the server, you must specify DBNull, not null. The null value in the system is an empty object that has no value. DBNull is used to represent null values.  

If the application specifies the DbType, the value is converted to that type when the provider sends the data to the server. DbType can be inferred from Value though it is not explicitly set by setting Value. 

Value (MSDN) 

DBNull (MSDN) 

TAdoDbxCommand 

TAdoDbxDataReader 

DbType 

Direction