By pre-filling certain form fields, it helps reduce the time that customers spend filling your form. Therefore, it's more likely that they complete your form which will increase the form conversion rates.
Let's take a look at how to pre-fill forms in Blue Form Builder, a simple yet powerful Magento 2 form builder extension.
https://www.magezon.com/magento-2-form-builder.html

- Open the builder section in the form edit page. Hover over an element and you'll see a bar. Move the mouse to the element name and click Edit in the drop-down menu to open element settings.

- Open Advanced tab and find Default Value option where you can choose the value you want to pre-fill from a drop-down list. There are 3 types of pre-filling values related to Customer, Page and Product
For example, if you pre-fill Email form field with [customer.email] value. Then when a logged in customer accesses the form, this field will be automatically filled with their email address.