In my opinion, people often use the concept of opinions as an excuse to avoid thinking and communicating deeply. For example, I could get away with not bothering to provide this example, because I said 'in my opinion'; I don't need to provide any reason or evidence for my point, because I have stated that it's an opinion.
On the other hand, sometimes people use the word 'opinion' to dismiss other people's arguments without due consideration. If you believe the Earth is flat, you can shut down someone who wants to convince you that the Earth is round by saying, "that's just your opinion." Even if someone presents a compelling argument and plenty of evidence, it's all too easy to reframe everything they have said as 'just an opinion'.
When it comes to trivial matters, of course, people shouldn't have to justify their opinions. They can say "let's agree to disagree." But what if they also disagree on what is trivial and what is serious? The seriousness of a problem is also a matter of opinion. For instance, many atheists and theists regard "does God exist?" to be a very important question, and may spend a large part of their lives arguing about it, regarding it as something more than 'just an opinion'. On the other hand, an apatheist doesn't care whether God exists or not, so will probably be quite happy to regard the question as 'just a matter of opinion'.