When an email is sent to the contact, the Update Client Details app creates a task which is also added to the activity history of the contact, as shown in the image below.
In Lightning -
In Classic -
Additionally, there are the managed fields of the Update Client Details app on the Contact object, which get updated when an email is sent to the contact, as shown below-
This helps you to keep track of when the contact has received the form previously and by which e-mail. You can add these fields into your contact layout for better understanding.