For Example, in a package we import the data from various sources and the columns have X data type but the data type of destination columns have Y data type, in such cases we need Data Conversion Transformation.
We place Data Conversion Transformation between Source and Destination so that it converts the data type of Source Columns to make it compatible with Destination Columns.
Let's take a real life example where we will import a Flat file to SQL Server and we would require Data Conversion Transformation
Import Flat File to SQL Server using SSIS
In the above example, there are two columns Data and Amount which have incompatible data types and we have used Data Conversion Transformation to make them compatible.