The earth spins at the rate of one complete revolution every 24 hours. It hasn't always been so:
If our studies tell us that we could modify the spin rate, it might be a practical way of changing the amount of sunlight that hits the earth each revolution (day).
One could imagine that our research might tell us that it isn't practical to change the orbit or even the tilt. Yet, we might be able to modify the spin rate.
We might be able to modify the spin rate to help us defend against global warming or the onset of a new ice age. The daily dose of sunlight we get and the daily amount of cooling we get from the nighttime are factors which we might bring under our control.
One could also imagine that we could modify the spin rate, at least temporarily, to help control where a threatening asteroid might land.
Outside of the existing threats that we on earth face from hurricanes, tornadoes, monsoons, droughts, tidal waves, and earthquakes, the other major threats are longer term: global warming, onset of an ice age, and impact from some large space object. Affecting the spin rate might give us some protection from each of these potential future threats.