Several times it becomes mandatory to use a tripod and taking pictures from the hand is just not possible. Many photographers like professional wedding photographers discard the use of hand entirely and shoot every picture with the help of a tripod. Wild life photography, shooting moving objects, some weather subjects etc certainly require a tripod. It is to be remembered that when the shutter time is very slow and is less than 60. It is easy to use a tripod but inconvenient to carry them along. One just has to attach a camera to the head and this head can be rotated or panned to face any direction. Then adjust the legs with screws provided to suit your purpose and get on with clicking the picture.
Its function is to stabilize a camera. Sometimes you need to take pictures with long exposure. With all due respect to your muscles, hands will shake a bit and small vibrations will happen anyway. The tripod copes with this. They can cost from a few dollars to grand or even more. So, the variety is big and everyone can pick the Best tripod they can afford.