What is the reason behind it or was it just a gimmick?

http://flavorwire.com/wp-content/uploads/2012/03/Kojak.jpg

I think the idea was that Kojak was substituting lollipops for cigarettes at times, as a means of cutting back on or quitting smoking. I don't know if Savalas himself was trying to do the same in real life.

That was his trademark!