If it has less internal resistance it will generate less heat for the same amount of work. Semiconductors are constrained by the maximum junction temperature[1], and CMOS logic chips like modern processors primarily consume power when switching.
So there's a thermal limit to how fast they can switch. Thus having lower internal resistance can allow you to clock higher (switch more often) before reaching the thermal limit.
However, when switching they act as varying resistors, and the amount of power dissipated in any resistor depends on the square of the applied voltage (P = V^2/R).
So if you can run your chip on a lower voltage, it will dissipate a lot less energy when switching, thus allowing you to switch faster before being thermally limited.
When overclocking, you ask the transistors to switch faster. To turn a MOSFET[3] transistor on or off, you need to transfer a certain amount of charge (aka electrons) into or out of the gate. The faster you can transfer the charge (more electrons per second ie the higher the current) the quicker the transistor turns on. And the way to do that is to increase the voltage[2]. So overclockers tend to bump up the voltage when going for the super-high clocks. But as you see from above that causes a massive increase in heat, thus requiring significantly more cooling.
I'm no semiconductor expert though, so I might got some of this wrong, but I hope I got the essence right. Also I'm not sure if the impurities and such that lead to the variability of the internal resistance is related to the ones affecting the ability to undervolt. Is it mainly the threshold voltage or?
There are other aspects as well though, I assume the parasitic elements[4] play a role as well, especially when overclocking.
[1]: https://en.wikipedia.org/wiki/Junction_temperature
[2]: https://en.wikipedia.org/wiki/Ohm%27s_law
[3]: https://en.wikipedia.org/wiki/MOSFET
[4]: https://techweb.rohm.com/knowledge/si/s-si/03-s-si/4873
[4]: https://techweb.rohm.com/knowledge/si/s-si/03-s-si/4873