The credit calculator allows you to calculate the cost of the assessments you want to make and acquire the credits. The calculator is not used to create projects, only to calculate and acquire credits, later you will see in the "project creation" that you can also acquire the credits for a specific project at the same time you create the project. If you are planning to do several projects or different types of assessments with different costs, instead of having to acquire the credits for each type of assessments, you can acquire them with the calculator in one go and have them available when you are going to create those projects or assessments.
You can also purchase a number of credits of your choice by writing down the amount of credits you want.