It's always recommended to also use Upwork's messaging system to keep track of the progress as proofs for both employer and freelancer. It's inevitable to have disputes in the middle of the project and the messaging system will help leverage it when Upwork's team investigate the issue.

(As far as my experience, when you are dealing outside their platform, they will not be able to add those as evidence when they investigate a dispute so that would be a bummer for the employer or freelancer about it)