What are the drawbacks of operating infrastructure manually?

Manual operation is costly, slow and hard to scale, error-prone, and depends on human-written documentation that is incomplete, ambiguous, untestable and quickly out of date.

Each drawback follows from a person being the execution engine:

• Cost and time. Every change takes a person's working hours, so doing ten times more means paying for ten times more hours. That is why it scales badly.
• Human error. People mistype, skip a step, or do step 4 before step 3. On a hundred devices, some will end up configured differently.
• Documentation decays. A runbook written for humans is often vague ("configure the firewall appropriately"). Nothing checks it, so it silently drifts away from what is actually deployed.

Automation fixes the last point in a neat way. The script is the documentation, and because it runs, it cannot drift from reality without failing.

There is also a business angle. Speed and agility let a company try out opportunities before competitors do. DevOps expects developers to deploy and run their own applications in production, so they need to automate those tasks just as they automate coding, testing and staging.