One type of data preparation is data enrichment. It uses external and internal sources to improve and complete records from incomplete sources.
Suppose you have customer names in your CRM. However, to provide better customer service and do better marketing, you want more information about them, like headquarters, addresses, information on founders, etc. Procuring external reference data from a data provider like Dun & Bradstreet and enriching your CRM data with theirs is how you would do that.
You can also enrich your data internally from disconnected data sources, taking data from one and using it to complete another.
You use customer phone numbers from one source and DOB from another to create one master customer record.
Like any type of data preparation, data enrichment is essential because it makes your data more usable and saves data scientists time.