Why should projects be completed?
From Planfix
It's a good idea to complete a project with no more planned active tasks. When you do so, the task will get the status "Complete," but it will still be available in your Projects section.
The main advantage to completing a project is that complete projects won't be displayed in lists when you create a new task or in other places. This makes life easier: you'll have a smaller number of projects to search through.
You can return a completed project back to "Active" at any time (in the project card).