A test is a small program that runs another program and checks whether what comes out is what should come out. On an invoice with a discount and a reduced tax rate, this one figure has to appear at the bottom. A test holds that fast and speaks up as soon as somebody changes it.
Laravel brings everything needed. An application can be started up completely, a call sent and the answer checked, with a database of its own that is empty again afterwards.
Where tests are worth having first
Not everywhere equally. They bring the most where business logic sits that has to be right: prices, deadlines, permissions, billing. Those places rarely change and hurt properly when they go wrong.
They bring the least on screens that are still moving. A test that breaks on every visual change gets switched off after a fortnight, and then stands around uselessly.
Tests deliver the most value together with CI/CD, because they then run by themselves on every change and nobody has to remember them.