In this game, I am the dealer and sit in the South position. The score is 9 - 6. A jack is turned up and first seat(W) has a junk hand with no stoppers. It was the type of hand that screams 'Look at the score, Call a block'.
Here is how the game played out. The East/West team was ahead 9 -4. The cards changed and East/West was euchred. Now the score was 9 to 6. The hand showing came up next. Bidding went pass, pass, pass and I think to myself "how sweet is this", as I call "alone" in diamonds. The East/West team lost the game due to an unwillingness to block. While the old saying goes, "nice people finish last". This isn't about someone being nice or not. It is about winning or losing. Euchre is a competitive game. You should always play to win.
Again, the dealer sits in South. This is not as obvious as the one where a jack is turned up. Still, the score of 9 - 6, says it all. As in the hand above, West has no way of stopping a lone. A block would have assured E/W that they would still be in the game for one more hand.
Another use of blocking is to protect a large lead. Here is what happened in a game I recently played. We were ahead 9 to 4. The dealer sat in the West position and turned up the jack of clubs. I had already played a couple of games with my partner and he seemed like a solid player, I was not worried when he passed; I assumed he had the lone stopped. I was wrong! As I reviewed the game, it seems my partner was waiting to play next. He must not have thought about the fact 'if there's one lone, there could be two'. The dealer went alone and made the bid. On the next hand, I bid with three trump and off ace. The problem was that third seat had four and was sitting behind me. As you may have guessed, I was euchred. As a team, we turned a six-point lead into a lost. Had my partner blocked at 4 - 9, we would have had two or maybe three more chances to make that last point.
If you have a large lead, don't give your opponents a chance to catch up. Even though the card distribution had favored your team for the last couple of hands, they can change very quickly. To give your team the best chance of winning, always protect your lead.