Advantages of Using the Plesk Control Panel
The Plesk Control Panel is an application that manages most aspects of a web hosting environment.

The Benefits of Using the Plesk Control Panel
Plesk interfaces with

IIS and web services
Email services
FTP services
The database servers
DNS services

It also

Manages client accounts
Manages domains
Sets up and manages the file structure
Sets account limits
Sets user permissions

While many of our customers are more than qualified to manage their domains directly through IIS, this becomes very difficult to support. There are multiple ways certain tasks can be performed via IIS configurations. Plesk performs that same task in just one way. It is much easier to provide support when things are done consistently.

For these reasons, we ask our dedicated server customers to do everything they possibly can through Plesk. Set up email accounts through Plesk which applies those changes in SmarterMail. If you create accounts directly through SmarterMail, when Plesk runs its nightly tasks, it will note that there are email accounts that it knows nothing about and will delete them.

Create your clients (if needed) and domains in Plesk. It will set up the necessary folder structure, apply limits and set user permissions appropriately.

Generate and apply SSL certificates through Plesk. Create portal aliases and subdomains through Plesk. The list goes on and on.

The Few Tasks We Commonly Perform Directly in IIS

Recycle an App Pool - While this can be done through Plesk or through the Control Suite, nothing is as fool-proof as going directly to the source.
Creating or modifying wildcard mappings
Creating or modifying handler mappings
Switching from integrated to classic pipeline mode or vice-versa

In summary, do manage your server as much as possible through Plesk. Plesk and the Control Suite can make managing your server much, much easier.