In half the cases, it's just noob behavior, especially if you're playing a carry.
And what a morale boost for the team! The ally won't just type "thx"; they'll appreciate the sacrifice, understand their importance to the team, and will put in 10-20% more effort to return the favor and carry the game. And with them, the entire team can be charged with such selfless spirit.
But this is only with a normal team. We never save idiots if we ourselves might end up getting beaten – that's the main thing.
