It forces mushrooms to grow in a straight line by not providing any other available squares. Now I realized that in addition to playing Mushroom Racer, this kind of farm could also be used as a random count down timer like this:
You'd place the first mushroom and let it spread down the track. A block update detector would react when a mushroom finally grows at the end and it would set off some device (maybe also causing water to clear all the mushrooms but the first one to restart the timer). Although it's impossible to predict how exactly how long the mushrooms will take to grow, you could control the average amount of time by how long the track is.
I could imagine this being useful in any kind of machine where you want to activate it after random long intervals. Or it could just be used as a fun kind of timer. "Complete the challenge map before the 100th mushroom grows!"