Studs keep the building structure in place and have enormous strength. They provide solid support to any item you want to attach to the wall. For example, you could attach heavy bookshelves to the wall studs so they don't topple because of the weight of the books you put in them. You could also use studs to keep other items in place, including towel bars, picture frames and mirrors. Even during earthquakes, drilling these items into the studs could keep them in place.
The easiest way to find horizontal wall studs is to use either a magnetic or an electronic stud finder. A magnetic stud finder looks for the nails that keep the drywall and the studs together. The electronic stud finder costs more but it scans for areas where the wall density is thicker. Depending on the particular model you use, the electronic stud finder may beep or flash a light when it finds a stud. Once you find a stud, move the stud finder sideways to confirm that the stud runs horizontally and not vertically.
If you don't want to get a dedicated tool to find horizontal studs, you could use other methods. Shining a light over the wall surface helps you spot tiny indentations where nails go through the drywall and into the studs. If you find several indentations along the same horizontal level, that would be where a horizontal stud is. Alternatively, you could drill a hole through the wall, insert a long wire and probe upward and downward to find a horizontal stud. If you knock on the wall along a vertical line, you might hear a different pitch where there is a horizontal stud behind the wall.
When you have found a horizontal stud, you have finished the difficult part of the project. Mark the location of the horizontal stud and then try drilling a shallow hole there with a small 1/16-inch drill bit to confirm the presence of the horizontal stud. If you hit the horizontal stud, the drill bit would remove some sawdust from it. You can then drill a larger hole into the horizontal stud if you want.