We're all familiar with two reasons to define terms. One: because you're speaking to someone who doesn't know what the word means. And two: because you're speaking to somebody who misunderstood the particular meaning of the word you intended.
But philosophers purpose a third option, one that gives definitions a much bigger role than we might be used to in every day life. According to philosophy we define because our concepts are rough around the edges. That is, we want to talk about what a thing is and what it isn't, in absolute terms, but our concepts aren't well enough understood to do this. 'How can we make any real progress talking about justice if we actually don't understand what justice is and what it isn't yet?'
If you weren't used to philosophy and you peered in to take a look, all this back and forth about what free-will is and what it isn't, or if ethical statements express beliefs or commands, or if knowledge is justified true belief (or if that requires either amending or additional conditions), and everything else philosophers waffle on about, would all seem rather pointless. It's only not pointless when you understand how what they're talking about relates to real problems and solutions to those problems.
Philosophers have a specific definition of 'definitions' and that is: to analyse the necessary and sufficient conditions of a concept. So when philosophers define free-will, say, they aren't being pretentious, rather they're trying to formalise a concept of free-will that resists criticisms from determinists where other accounts of free-will have failed. And when Gettier wrote about 'justified true belief being necessary but not sufficient for knowledge', the implication wasn't some minor curiosity about words, but an error in our concept of 'knowledge' that could lead to re-evaluate our entire treatment of what we know given fallibility.
Real work is done with philosophers definitions.
What the philosophers have to be careful of, however, is getting too into definitions, and forgetting all the other ways we have to make progress in philosophy. Actually you can learn a lot about ideas like 'good' and 'bad' in ethics, even if you aren't really sure what 'good' and 'bad' exactly mean. It's good to bring up definitions if you notice a problem with one, or if you think one will solve the problem, but bringing one up to stop a conversation thread only ever stops potential progress.