@Lightning a person’s truth is their opinion yes. And it is important for them to speak their truth, aka true opinion. It may not be the whole truth, but it’s not detached from it either. Most statements or positions have degrees of truth to them.
Why it’s important. If you’re wrong, you’re more likely to find out, if you’re right you’re more likely to teach someone. In general, there can be an actual improvement.
Of course you can try very hard to come up with cases where saying the truth was damaging, and lying was beneficial at least in the short term… but overall I think the opposite is much more likely.
On that note, there are ways of telling the truth that avert catastrophic results—by working on the delivery.