I'll include the Azure compute services architecture guide as well as some other links at the bottom, but the short answer to your question is that DevTest Labs is designed to enable developers and testers a way to quickly spin up and tear down development and test environments. DevTest Labs is not intended for production environments, which is typically where you would choose an Azure compute service.
DevTest Labs
Used for quickly developing and testing using self-managed virtual machines (VMs) and PaaS resources:
- Rapid setup and tear down of VMs using custom images and artifacts
- Ability to claim a pre-created VM (self service) without having to create a VM for each individual
- Minimized number of steps necessary to create VMs and PaaS environments (5 steps for VMs, 3 steps for PaaS)
- Integrate with existing tools (CI/continuous integration, IDE/integrated development environment, automated release pipeline)
- Auto-shutdown and auto-startup available on VMs
- Policies on number of VMs, size, images, and more, as well as cost tracking
- Lab Services: Ability to run a managed classroom lab.
More info: https://learn.microsoft.com/en-us/azure/devtest-labs/devtest-lab-overview
Compute Services
Frequently used for production environments. Provides VMs, VM Scale Sets, Kubernetes, serverless functions, microservices, cloud apps for web and mobile, containerized apps, and more.
https://learn.microsoft.com/en-us/azure/architecture/guide/technology-choices/compute-decision-tree