Just FYI, “OH FORGET THIS TERRIBLE BOARD” statistically happens more often if you use mono teams.
But mostly flee is good to protect your gains from further losses.
When I am down to 1-3 heroes with low health, and see no good matches, maybe none that will charge my heroes mana, and ones that cannot possibly start a cascade…well, I often know there’s NO WAY I can win or even do more damage at that point.
And as others have pointed out, you can lock in damage to heroes in war … or lock the 1-2 dead heroes into your raid chest. And if Alby or MoNo is about to fire, or a healer or heal aid as soon as you make a move, you are better off fleeing.