They are sometimes called taps in the RF industry. It's for when you want have a monitor port for a signal without disrupting the original signal too much. You'

They are sometimes called taps in the RF industry. It's for when you want have a monitor port for a signal without disrupting the original signal too much. You'll still lose a little bit of power on the original signal but this will only be as much as 3dB for a splitter (where both outputs are equal in power) or much less for something like a 15dB coupler (the monitor signal is 15dB less than the original output). Since it's impossible to measure an RF signal without stealing at least little bit of power, there is always just a little bit of loss on the original signal. There are similar devices for fiber optics where they bond a fiber to the original so some light escapes the original fiber and goes into the second bonded fiber. This can even be done in the field on in-use lines where a tap fiber is wrapped tightly around the main fiber and bonded with a torch. You can even tap a fiber line by just bending it enough and placing a detector at the bend since some light will escape there. Something similar can be done for light where a diagonal piece of glass sits in front of one camera and another camera sits 90deg pointed at the same glass. Some light gets through the glass but most reflects so one camera sees a slightly darker image and the other sees a much darker image.