I assume that you are happy with the idea that, when a spaceprobe is launched towards Mars, once it's rocket engine gives it enough velocity, it can be turned off, as there is nothing in the vacuum of space that slows the rocket down. The probe is provided with inertia, which requires a force to change it.
You can apply the same reasoning to the rotation of the Earth. After it formed from the accrection of material and given it's initial velocity, it would in theory continue to rotate with the same velocity because of conservation of angular momentum. The Earth was given an inertia, around its axis, which requires a force to alter it.
In practice, there are forces acting to change the inertia of the Earth. We have to take into account the effect of the moon, which through the action of the tides (as well as on the solid material) of the Earth, is gradually slowing the Earth down.
From Initial Velocity of the Earth
Scientists think that a large object, perhaps the size of Mars, impacted our young planet, knocking out a chunk of material that eventually became our Moon. This collision set Earth spinning at a faster rate. Scientists estimate that a day in the life of early Earth was only about 6 hours long.
The Moon formed much closer to Earth than it is today. As Earth rotates, the Moon's gravity causes the oceans to seem to rise and fall. (The Sun also does this, but not as much.) There is a little bit of friction between the tides and the turning Earth, causing the rotation to slow down just a little. As Earth slows, it lets the Moon creep away.
You also need to note that the inner core of the Earth rotates faster than the surface, Inner Core
The inner core rotates in the same direction as the Earth and slightly faster, completing its once-a-day rotation about two-thirds of a second faster than the entire Earth. Over the past 100 years that extra speed has gained the core a quarter-turn on the planet as a whole, the scientists found. Such motion is remarkably fast for geological movements -- some 100,000 times faster than the drift of continents, they noted. The scientists made their finding by measuring changes in the speed of earthquake-generated seismic waves that pass through the inner core.
This inner material has a different inertial value than the slower inertial value of the outer regions and surface of the planet.