To do their work in the ring. Arguello immediantly springs to mind. Almost all of his big fights ended late. Very patient, very methodical fighter. Emile Griffith was a distant fighter for most of his career and extended fights the full distance.
Ali's a good one that I hadn't thought of for some reason. Late stoppage against Lyle, Frazier, seconds away from having Shavers out of there in the 15th...
Zuri Lawrence. http://www.boxrec.com/list_bouts.php?human_id=015802&cat=boxer Look at how many of his 23 wins are by KO. He needed time, definitely.
Foreman he stopped in eight. More of a case of "I need time to let this guy punch himself out" as opposed to "I need time to break this guy down". Foreman was literally completely broken BEFORE that fight. He had the entire continent of Africa against him and was so paranoid he thought his ****ing food was being poisoned and he was being cursed by witch doctors and ****. He got right up off the mat and walked to his corner after being knocked down. Beaten man.
Ali's plan was to outlast Foreman, though, however long it took..it just happened to only take eight rounds He needed time, just not as much as some other times.
Michael Spinks, Carlos Palomino and Ken Norton. Michael and Carlos could hit like a ton of bricks, yet Michael did not produce a single opening round stoppage during the 1980s. (In fact, David Sears and Marvin Johnson were his two quickest victims, going in three and four rounds respectively.) Palomino's only first round victim was Arrendondo, during his geriatric 1997 campaign. Norton only did this twice, but Bobick makes everybody forget that Ken was an even slower starter than Frazier. (Duane himself could hit, but was very candidly a self admitted slow starter.) ThinBlack's mention of Saad Muhammad is to be credited as the inspiration for my choices here. I wanted to go with punchers who enjoyed a reputation for power. I purposely omitted Mustafa Muhammad, because Eddie had eight opening round stoppages throughout the entire course of his career, and because temperament rather than necessity was responsible for many of his bouts lasting more than a few rounds. Kenny, Mike, Matt and Carlos really did need time to get going though.