Here are three useful phrases:
this is useful information
this is not the conversation you need to have right now
I appreciate your presence/input
They are all useful in cases where you need to reframe a situation. Which you will need at many points in your life. Especially in situations where things have objectively gone to shit, and the temptation to give in to despair looms large in the general ambiance of being. It might seem a small thing, but each of these phrases can help stave off the despair until there is ample and proper time to go through the process of feeling it.
Let's say something has a critical existence failure. It breaks down, and no amount of rebooting or replugging will unbreak it. It has gone to shit, and you find yourself in a situation where you have to keep moving forward without it. What to do?
It's tempting to think of all the things you suddenly cannot do, and heap all the negative consequences of this sudden inability onto a huge heap of despair. And then compound the matter by adding on the negative consequences of these negative consequences, fueling the despair and then just keep going.
It's tempting, but not very useful.
One way of staving of this temptation is to look upon the situation and state "this is useful information". Why is it useful information? Because it is brutally relevant to your lived experience, and you can use it to approach a solution. The car broke down - which is useful information, since you can now coordinate your thinking around the logistics of getting around without one. The computer fried - which is useful information, on the same general principle. Your inner monologue can refocus from general despair to goal-oriented problem solving.
The same goes when you're not alone in facing this sudden turn to shit. Social situations depend heavily on the moods of those involved in it, and you can reframe it by jumpstarting your problem solving skill. Especially if you materialize it in your communication, for instance by asking what would be needed to get past the immediate situation. You know what's wrong, and can both act on this knowledge and communicate the state of things to others.
That is useful information.
As to the next phrase, it applies in situations where someone is acting in a way that does not bring them closer to what they want or need. As in, say, someone picking a fight with someone that is not involved with the problem or feelings that caused them to pick a fight. Which is more common than one might think, and reminding the person in questions to pick better battles gives them the opportunity to back up, apologize and approach the situation in a more constructive manner.
Granted, given enough anger, being reminded that the conversation/argument they are attempting to initiate isn't the one they need to have will most likely make them even more angry. But you've at least given them the chance to opt out, and, more importantly, acted on the real issue rather than simply gotten into a fight for no reason.
The third phrase, then, is as general purpose as it gets. It has the useful effect of acknowledging the contribution of the other in an unequivocal, non-confrontational and direct manner. Especially in situations where you've received critique of some kind and want to avoid debating it. Just state that you appreciate the input, accept it as given and move on. The input is appreciated, and it has helped you further towards your goal.
That is useful information to convey.
The general usefulness of these phrases does not lie in their literal recitation in real situations. Rather, it lets you think about and reframe situations as they occur, and move forward with a better understanding of what's going on. Things turned to shit - that's useful information, let's act on it. Something turned to shit for someone else - that is useful information too, and lets you have a more appropriate conversation about it. Someone sticks around and helps you with the general shittiness of things - that is indeed an appreciated input.
Think about it. It will help you down the line.