When to use a Scatter Plot
You are testing a relationship between two numbers
Ad spend against revenue, price against rating, house size against sale price. If both variables are numeric and you want to know how one tracks the other, this is the default chart.
You need to find outliers
Points far from the crowd are obvious on a scatter plot and invisible in a table or a bar chart. Analysts use them for exactly this: spotting the record that breaks the pattern.
You want to compare groups on the same two axes
Colour the dots by category — region, segment, asset class — and you can see whether the groups occupy different parts of the space or overlap completely.
Your data has too many rows to plot as bars
A bar chart with 400 categories is unreadable. A scatter plot with 400 points reads fine, because the eye reads the cloud rather than each mark.