I keep seeing killers with Unrelenting and Save the best for last... Why are those perks so popular? I never use them as a killer
Unrelenting reduces the cool down for attacks you miss (up to 30%), while Save The Best For Last reduces the cool down for attacks you land (up to 40%).
Both perks together make it so all attacks have a reduced cool down, so you (as killer) can attack more often.
Having faster attack cool down messes up Survivor's reliance on missed attacks. It allows for mind games and catching people off guard.