I cant seem to find any information why we need to create a Collection.php file inside ResourceModel and what is the purpose of collection.
With a model and resource model, you are having everything that you need/want to fetch and save from/into the database.
However, there will be some situation when you’ll require to fetch multiple models of a particular type. To solve this problem, every CRUD model in Magento 2 has a corresponding resource model collection.
A collection collects individual models. It’s considered as a resource model because it builds the SQL code which required to pull the information from a database table.
For more information you can refer this site