For example, by default, the SYSTEM account runs the VisualCron Server service. It has no network access rights by default. By using a Credential you will be able to gain the network rights of the user in that Credential. You have the option login locally and load the profile and if the whole process is loaded it will run _as_ that user.
This has changed some versions ago the following way:
1. Load profile option was moved from Execute Task to the Credential
2. since the move, other Tasks can use this feature (which wasn't available) - only network rights was possible to gain - not a full "run as"