When your car has a single-speed transmission, your engine may be geared for acceleration (quick start-up), top end (the top speed you could attain), or some point in between. Normally, you can't gear for top end and acceleration at the same time, just like a seesaw can't have both ends in the air at once. With a two-speed, however, you can cheat a little and slightly broaden the gearing range. When you are in first gear, your two-speed may be geared for acceleration. When you shift into second, you can be geared more for top end.
The two-speed gives you more leeway with gearing adjustments. For instance, you would never reach top end in track sections full of tight turns. Those sections favor quicker acceleration. Top end gearing is useful in those sections where you can go all out. With a single-speed transmission, you could gear only for one section or the other, possibly losing ground in the other sections.
By increasing or decreasing the spring tension in the two-speed's clutch shoes, you can change the shift point from first to second gear to take advantage of this broader gearing range at strategic points in the race. So when you hit a high-speed section of track, the two-speed will shift into gear for top end, then allow it to drop down again to take advantage of quicker acceleration in the turns. Many people buy two-speeds for this reason--to go faster.