Learn English – What’s it called when you buy something you don’t really need?

This is formally termed a "discretionary purchase".

For example, from the synopsis of the book "Why People Buy Things They Don't Need"

Focusing on the "whys" of spending, Danziger has meticulously profiled customers in more than 30 categories of discretionary spending through research based on surveys, interviews, and focus groups from a variety of people who make discretionary purchases.

And later, in the content of the book:

Marketers can use these insights to position their products strategically along the discretionary purchase continuum, playing to the various justifiers that ...

One sub-type of discretionary purchase, when done on-the-spot due to a sudden urge without a lot of forethought (think buying a magazine at the supermarket checkout line) is called an "impulse purchase"