Frequently there is a place with LOS but it is too far form the computer to be usable. In some cases you LOS is too high above the house of building to make the installation reasonable without the installation of a tower which might be unacceptable for your home or your neighborhood. One of the most common problems with LOS is getting around trees.
Trees contain a large amount of moisture which absorbs microwave energy.
A thick cover of trees can completely stop the wireless signal unless you are less than a mile from the tower site. In many situations you may avoid trees by not trying to go over them or through them. Since the height of the antenna is not important, you might find a LOS under the trees. All you need is a hole about 3 foot in diameter which the antenna can see through to see the tower. In some situations you may have a complete blockage of trees as in the picture below but the row may be thin or only contain a few trees that are in a line.