dahlias like the ground temperature to be 60 degrees or more or they might just sit there & rot. If you start them in pots or flats you get a head-start on the growing season & you get earlier blooms. You can divide them down to one tuber with a start so they won't be as bulky to pot up. You get a nice big plant from one tuber.
The bigger the clump you plant, the less the plant does to reproduce itself. So if you plant a big clump your dahlia will put less effort into blooming & tuber production & just live off of the old tubers. This is why, when people leave the clump in the ground for a lot of years, the plant gets less prolific & spindly.
Ellie

I have great luck with starting mine inside. I get much earlier bloom than other people in my area (Chicago) who wait. Once they are "awake" and well leafed out being put in cold ground does not set them back.

Thanks for the replies Ellie and linnea56.

I will further divide the clumps that have only eyes and leave the clumps with sprouts alone. In the fall, I can revisit the clumps that were not divided.

Greg
PS I grew up in Rockford and miss the three seasons (not winter)