Let's imagine that you want to run a report that shows the names of all of your customers and their orders.
To do this, you need to combine data from two tables: Customer and Order.
If you use an Inner join, it will only give you records where there is a matching customer and order.
This means that you will miss out on any customers who haven't placed an order yet.
An Outer Join solves this problem by combining all the data from both tables, matching where it can, and giving you the rest of the results as nulls.
This allows you to see all customers, even those without orders.