MikeJ - Do-It Molds do not require a release agent of any kind to work properly.
That being said, smoking a mold or using a release agent can help in the release of the castings.
I was much like Ctom in regards to never needing a release agent and have been pouring that way for a long time but I just recently started using more of the casting release spray. I do this for three reasons - The biggest reason is the spray helps give the castings a much smoother finish on the jig or sinker. The second reason is it does indeed help the castings come out of the mold much easier than without it. Lastly, as mentioned by others it does help the flow of the lead as it moves through the cavities.
Again, the main reason for me to use the release agent is the smoother finish I get on the jigs or sinkers I'm pouring. I've also found that on sinkers that have the size of the weight indicated on the casting itself that the release spray greatly affects your ability to read that weight. Sinker's sizes poured in a mold that's been sprayed with release spray are much easier to read than those poured just out of the mold. I've used similar molds that have been candled/smoked but did not have the same results with the exception of noticing a difference in helping remove the cavities.
Also, the release agent does not transfer over to the jig so there is no clean up needed before painting. The release agent is also easily removed from the mold with rubbing alcohol should you decide to do so.