Isolation describes the ratio of signal appearing at port 2 to the incident power on port 3 when it is excited and port 1 is terminated. The directivity of the directional coupler describes the difference between the isolation and the insertion loss. When directional couplers are used in headend combining networks, isolation is very important. You are usually injecting signals on port 3, with port 1 taking signals closer to the output of the combining network, while a number of signals from other processors and modulators are entering on port 2. If the splitter has poor isolation, signals from port 3 will find their way to port 2 and from there into the output of the upstream modulators and processors.
Directional couplers are also the first stage of taps.
Most taps consist of directional couplers to remove signal from the distribution cable, coupling it to a splitter or series of splitters that divide the signal into homes. When you worry about the isolation from one tap to another, you are worrying about the performance of the directional coupler. If it has poor isolation, you get poor isolation from one tap to another. One problem arising from this is the coupling of local oscillator signals from one TV to another. All TVs have some local oscillator signal present at the antenna terminals.