It takes a certain amount of watts to raise a certain amount of water a certain temperature...doesn't matter if it's trying to do it all at once in a tankless situation, or over time in a standard tank. The biggest difference is in the peak load and the flexibility. In theory, a tankless could provide a continuous stream of hot water...a tank will likely not be able to keep up. But, if a tankless was asked to provide say 10gpm, it couldn't at least with a normal temperature rise, whereas, since you've stored that hot water, a tank would have no problems at all as long as you didn't ask it for more than it had stored up. A typical tank, no required maintenance. A typical tankless, at least annual or you'll notice degradation of operation.
They can make sense for a small public restroom that sees heavy use where there's no space for a tank, the volume is small but constant where a tank would take up room and not be able to keep up unless larger.
If you're space limited, have no other power source, and accept their limitations, they can provide hot water for you, so they do have a purpose. For those growing up with a large tank, it can take some accommodation to its limitations and maybe a change in lifestyle. If you live where the incoming water is warm all year, they can work better than if your incoming water is frigid with fewer limitations.
Expect utility companies to start to charge for your peak demand capability if they don't already, even if your actual average load isn't very high. The whole grid works better when the load is fairly constant. Sharp spikes can mess up the network. Tankless electrical, if widely adopted, could disrupt things.