Multi-tier architecture provides a general framework to ensure decoupled and independently scalable application components can be developed, managed, and maintained separately.

Presentation Tier

Logic Tier

Data Tier

These layers can be scaled independently, and often can be managed by entirely different teams.

An highly available multi-tier application requires these layers to exist often in different availability zones or regions.

A single tier can be made resilient and run on multi AZ and with autoscaling, but you are not decoupling your services and you can't scale them independently