Luke Skywalker's disappearance in The Empire Strikes Back helped save the Rebels on Hoth. Even though they scored a major victory during the Battle of Yavin, the Rebels were still at a tremendous disadvantage during the Galactic Civil War. They were forced to go on the run from the Empire, eventually establishing a base on the remote world of Hoth. The ice planet was a hazardous place to call home due to freezing temperatures and the threat of wampas. Even Luke wasn't safe, as he was captured by one of the creatures in The Empire Strikes Back's first act.
When learning Luke had yet to return to Echo Base, Han Solo took it upon himself to find his friend. Initially, Han inquired about using one of the speeders to look for Luke, but was told the engineers were having trouble adapting them to the cold. That forced Han to set out on tauntaun instead, since he knew he didn't have the luxury of time because Luke was in grave danger. However, due to Luke's disappearance, the Alliance expedited the process of adjusting their speeders.
To celebrate The Empire Strikes Back's 40th anniversary, the new book From a Certain Point of View: The Empire Strikes Back was published. The novel is a retelling of the film's events from the perspective of various characters. One chapter, "Rogue Two," is written from Rebel pilot Zev Senesca's point of view. There's a passage revealing how Rebel mechanics responded when Luke disappeared:
The rebel mechanics assigned the task of adapting the speeders to the cold started working round-the-clock shifts. Those speeders were the best chance, maybe the only chance, of finding the commander before he froze to death or succumbed to whatever other fate had befallen him out there. So now they didn't take breaks, didn't eat, but instead devoted themselves to laboring constantly until they found a workaround for the coolant problem that was keeping the speeders grounded. When Zev first heard that Hoth was so damn cold it was even freezing engine coolant, he thought it was funny. It didn't seem so funny now.
Had Luke not been captured by the wampa, chances are the mechanics would have taken a more methodical approach with the speeders. What's interesting about this is the speeders were an integral component in the Battle of Hoth, going up against the AT-ATs. Without the speeders, it's highly unlikely the Echo Base evacuation would have been as successful as it ultimately was, and the entire Rebellion may have ended that day. The AT-ATs were a truly imposing opponent for Alliance forces, and it was only the speeders' tow cables that could bring them down. The Rebels would have been helpless if they didn't have the speeders, and fleeing Hoth would have been a much more difficult task.
The Rebels always knew it was a possibility the Empire would find them on Hoth, but they had no way of knowing it'd be so soon after Luke was rescued. In that regard, Luke's disappearance was serendipitous, as it inadvertently allowed the Rebels to be more prepared for the eventual Imperial attack. Obviously, everyone involved with the Alliance would have rather preferred Luke came back from his patrol safe and sound, but in retrospect, most would probably agree it was ultimately for the best he didn't. It shows that things have a funny way of working out sometimes. If they hadn't lost their fearless commander for a short while in The Empire Strikes Back, the Rebels may have lost the Galactic Civil War.