Spinning up on prem means you have to already the servers in your colo ready and have to pay for spare capacity. Depending on your tolerance for latency (production vs non-production environments or asynchronous batch processing), you can operate at almost 100% capacity all of the time:

Lambda vs VMs (yes you can deploy standard web apps to Lambda using a lambda proxy)

Serverless Aurora vs Regular Aurora.

Tx instances vs regular instances

Autoscaling VMs.

Autoscaling DynomoDB

Etc.