"Your Pipedrive is only as good as the customer data fuel you pump in."
During a sales process, there is a lot of data that needs to be collected over time while working with an organization, a person or on a deal.
For example collecting contact person preferences on something, filling the birthday, marking down the source they were found from, or any other custom field that appear to be important for the company.
While working on a deal there might be some custom fields (date fields or monetary fields) that are not that necessary to fill for closing the deal, but important for some other departments in the company to provide the service later, for invoicing, integrations etc. Therefore it may happen the salesperson might forget filling them.
Getting that reminder might be crucial while adding a deal/person/org, it might be an issue later while editing a deal/person/org, and it might be an issue only while closing a deal.
Learn more: Important fields - project page