The earth spins because of how it was created.

Millions of years ago, it was created amidst lots of gas and dust, which formed a small mass. As the gas and dust kept collecting and the mass continued getting bigger, the gravity surrounding it increased too. The more powerful it got, the faster it attracted these particles and created sort of a spinning whirlpool. At one point the mass, called Earth, couldn't get bigger and the gas and dust particle whirlpool was at a minimum, but the Earth kept spinning and will continue spinning.

It's not so much that the Earth has to spin, but it does spin. Spinning mass has what is called angular momentum, which is a physical quantity like energy, mass, or linear momentum. Angular momentum can't be created or destroyed (also like so many other physical quantities), so the only way to stop the Earth from spinning is for the Earth to hand off its angular momentum to something else.

The Earth is slowly doing this, actually: the tides caused by the moon exert a torque on the Earth which slows down its rotation, while at the same time pushing the moon further out in its orbit from the Earth. During the Silurian period, about 430 million years ago, the Earth's year was the same length in terms of number of seconds or minutes, but had over 400 days, meaning that the Earth was spinning fast enough to make 400 complete revolutions in the time that it now can only do about 365