Why to group related tests

What are the use cases for grouping tests using TestNG groups or JUnit categories?

I usually groups by tests by function using JUnit categories: unit-tests, integration-tests, etc. But at my current team, we just had a conversation and the team decided we want to run all the tests all the time because they don't see any benefits for grouping tests.

So I'm curious to know if people here group their tests and why.