Arun Sethia and Arshad Ali explain when you might use a Spark notebook versus a job definition:
A Synapse Spark Notebook is a web-based (HTTP/HTTPS) interactive interface to create files that contain live code, narrative text, and visualizes output with rich libraries for spark based applications. Data engineers can collaborate, schedule, run, and test their spark application code using Notebooks. Notebooks are a good place to validate ideas and do quick experiments to get insight into the data. You can integrate the Synapse Notebook into Synapse pipeline.
The Notebook allows you to combine programming code with markdown text and perform simple visualizations (using Synapse Notebook chart options and open-source libraries). In addition, running code will supply immediate feedback, output, and progress tracking within Notebook.
Click through for the comparison.