I've been undervolting many systems for many years, primarily Linux desktops and some servers, and the primary benefit is that you get less heat output which means when running cpu-intensive tasks the temperature climbs slower so the throttling of the clockspeed kicks in later, so your phone will be faster in certain situations. If you take a phone which has been idle for a while and run a benchmark, and then immediately run that benchmark again, the 2nd time gets a lower result as the phone is still hot from the 1st. This makes drawing conclusions about settings really dificult but it illustrate that throttling from heat is affecting speed.
For most users their perception will be the phone runs cooler.
You do undervolt at each step in the processor's frequency, and each step is a trial+error activity, the throttling I mention means finding a stable under-volt at the higher frequency which is labour-intensive,i.e take the max clock, and undervolt it a little, run a benchmark which forces it to run at high clockspeed, and if it passes that test then run it again at the next step down in frequency. Once you've got the most stable top clockspeed, then do it progressively for all the other voltages on the way down.
In some platforms in Linux and Windoze, we wrote scripts which save the stable voltages and then undervolts a little and runs a stress-testing benchmark and if the system hung it wouldn't save the current voltages so the previous higher voltages were safer, stick that script in a startup script area and leave the compute to do many self resets, and you've calculated your device's voltage range. I wonder if someone has that done for Android??? For a laptop the FAN would run slower saving battery time and for laptops that would lead to say 20% better battery life but on a phone it won't make much saving as no fan.
Your phone will run most of its time (like 95%) at its lowest frequency, so for effort/benefit just focusing on dropping its voltage will gain the most in the phone running cooler.
Battery life improvement is marginal, if you look at your battery stats its down to your application settings and screen brightness, i.e. how you use and what you do with your phone. So if your battery life is bad, use your phone less!
I carry a slim USB battery, it is the $/effort/benefit the best thing you can do, $20 doubles your battery life, if you get one with a 1.5A-2A output in just a few minutes when the phone doesn't mind a battery attached, will dwarth every possible tweak and hack anyone can form in benefit.
Nexus 5 stock 4.4.4 +ElementalX rooted, Nexus 7 2013 stock 4.4.4 +ElementalX rooed, Nexus 10 CM 11 rooted, OnePlusOne stock rooted.