They needed some way to explain how the Millennium Falcon with a broken hyperdrive could make it from Hoth to Bespin Cloud City. What's your alternative?
That Bespin and Hoth are extremely close together. The backup hyperdrive creates an even bigger problem as the entire reason they needed to go to Bespin was because they couldn't escape the TIE fighters earlier in the movie. A feat they could have accomplished if a backup hyperdrive was available.
The Falcon's trip from Hoth to Bespin is a worse mistake than the line about parsecs from the first movie.
It's in the script that Han was bullshiting the "rubes" to try and impress them and get the job.
The nearest solar system to earth is Alpha Centauri, at 4.37 light years distance. The best case trip from Hoth to Bespin would be comparable. That means the Falcon should have taken years to make the journey without FTL.
If Bespin was in the Hoth system, (which the movie plainly says it is a seperate system) our solar system is 1.5 light years across. The journey would still have taken months up to a year and half.
And that's at light speed. None of the ships in Star Wars are shown to muck about with near-relatavistic speeeds. The trip should have taken decades or centuries.
Space is big. Any time a sci-fi show that wants to have any kind of versimiltude should do a quick google* check on distances before writing something involving space travel times. Or go JJ Abrams and just not give any fucks. Put all the planets right next to each other and eat your popcorn.
*Yes, Empire was written before Google. There were plenty of sci-fi writers and real astronomers who could have told them about FTL and distances between solar systems.