Workflows are routines that can execute tasks in your home. They are a sequence of steps that will be executed consecutively. Each step is an action in your home, like reading or writing a value. In comparison to automations, you must start Workflows manually, but they give you much more flexibility.
Scenes are quite unflexible because they must explicitly define every characteristic they control. Workflows are much more dynamic and allow you to describe which characteristics you want to control. For example, you can create a Workflow that turns on all lights in your basement. Every time you execute the Workflow, it will determine the relevant characteristics automatically. Due to this, you don't need to change the Workflow if you add or remove lights.
Changes Over Time
A Workflow isn't just activated like a scene or automation. The steps of a Workflow are executed sequentially and allow you to control your home over a timespan. E.g., Let a lightbulb blink or increase its brightness over time.
A Workflow can only directly control your home while it's still running. However, it can generate time-based automations that will be triggered later on. A typical example is turning on a device and creating an automation to turn it off again after a specified time.
Another way to utilize Workflows is to collect information. A Workflow can return a formatted text as output. You can freely define that output.