A transform map is a set of field maps that define the relationships between fields in an import set and fields in an existing table1. When importing data from a spreadsheet, creating a transform map is the step that defines where the incoming data columns will be written in the receiving table2. The transform map allows users to specify how to transform the source data into the target data, such as by using scripts, coalescing, or mapping assist1.
References
* Transform Maps - Product Documentation: San Diego - ServiceNow1
* Importing Data: Excel to ServiceNow - Finite Partners2

This is a best practice because it reduces the amount of data that needs to be imported, transformed, and stored in ServiceNow, and improves the data quality and performance of the system1.
ReferencesImport and export resources page