Just about everything else in my installation tools is wired, or wireless. Or has something else I could use instead. For example, I typically have at least 5 different tools for cutting, generically called a "saw." But there is only one plumb bob.
A plumb bob is used to to establish plumb, i.e. straight up and down. My most recent occasion to use it was when I needed to establish the point location on the ceiling, but referenced off the floor, 13' below.
You unwind the string until the pointed metal weight is just above the point being referenced. A helper slows the weight from swaying back and forth like the pendulum it is, until it's over the point. Then I can mark the point on the ceiling that is perfectly over the point on the floor.
In the project below, the position of the ladder was determined from the floor, but I needed to know where to cut into the angled ceiling at the top to secure the ladder. A level, even a digital one, would not be accurate enough..