There are many occasions when using multiple roles dramatically reduces risk.
For example, an individual who has access to a developer environment, as well as a normal user environment. I would not want someone with dev privileges in userspace, so would expect two separate accounts - one which only works in dev, and one which only works in user. In this example the activities logged may be different in each environment.
For another example, you may have an individual who normally raises payments in their area, but on certain occasions needs to produce reports across multiple payment areas. You may provide them with initiate payment access in one area, and view only access across all areas, or you may give them their normal account for the day role, and another one specifically for the reporting role.
If you have multiple roles available to an individual, you use a toxic combinations matrix to ensure the roles they have cannot be used to subvert 4-eyes controls etc.
It is not harder to trace the actions as long as you monitor and log all the activities which may present a risk.