I think that the most compelling reason to have an Instant Hot Water Dispenser is to save counter space. Other than that, I think it really is just unnecessary complexity.
You can use an Instant Hot Water Dispenser for instant beverages, instant noodles, very hot water for clean up, etc. The hot water comes out of a special faucet on your sink and the heating unit is mounted out of site under the sink.
Yes, it is fast and you don't need to carry your kettle to the water tap. However you can take a kettle to the table to warm up your tea, or to the stove to add hot water to the soup pot and you can't do that with an instant hot water dispenser.
If I am using the unit for very light tea, I can sometimes detect an off-taste that does not occur with my Braun Agua Express Hot Kettle. This is especially true if the unit has not been used for a while.
There is a half gallon tank with the unit. When you first turn it on (or after a power outage) you can hear gurgling and steam. This leads me to believe that the unit is using electricity to keep that small tank at a certain temperature - this does not seem very energy efficient unless you use the unit a lot. Also, if this thing wears out and needs to be replaced by a handyman or plumber every 7 to 10 years, it seems a lot cheaper and easier to use a kettle.
The owner's manual says that the unit can be adjusted to provide water from 160 to 210 degrees Fahrenheit (preset at the factory to 200); it can deliver 60 cups of 200 Fahrenheit water per hour. There is a one year in-home warranty.
In summary, if you drink a lot of strong instant coffee and have more money than you have counter space, an instant hot water dispenser might be right for you.