Don Knotts had a big vacancy to fill when Norman Fell and Audra Lindley skedaddled to star in their own spin-off "The Ropers".

Moving into the apartment complex in Season 4, Mr. Furley tried to be a strict landlord with firm rules, but it didn’t take long for his feigned machismo and slow-witted reactions to deliver some serious belly laughs. While the Ropers were surely missed, Knotts’ bumbling, Barney Fife-like character was the next best thing, fitting in perfectly with the show’s surplus of comical misunderstandings and double entendres.

https://tvline.com/lists/best-tv-characters-late-cast-additions-seinfeld-buffy-the-office/best-tv-characters-late-cast-additions-castiel-supernatural/

Mr. Furley was a great replacement (https://jacksonupperco.com/2015/06/30/the-ten-best-threes-company-episodes-of-season-four/#comment-48626) for the Ropers because Don Knotts was just such a funny guy in his own right. He as Ralph Furley, played this old hippie, wannabe-womanizer.