This is called stratification which simulates natural winter conditions. A lot of seeds go through an embryonic dormancy phase, and generally will not sprout until this dormancy is broken.

So putting your Primrose seeds in the fridge for a length of time mimics the winter cold that they would go through outside and gives them a far better chance of germinating.

I have grown many, many primula from seed and have never stratified them. Whatever time of year I obtain the seeds they are sown immediately and the pot left in a shady position out of doors.

And if you do put them in the fridge, make sure it is not too cold. The salad compartment at 40f is cold enough.
Agree with Bulbaholic though.

I must admit that I was unsuccessful in germinating primrose from seed even though I gave them the cold treatment. I found that when I was given a few plants by a friend they self-seeded naturally and have spread like weeds throughout my front garden.

Try a little experiment Leaper, plant half of them after putting them in the fridge as directed and plant the other half of the seeds without stratifying them. I would be interested to know what the outcome is.

Wherever you sow them, keep them as cool as possible out of the sun. Primula seeds hate the heat. Ive never had to stratify them either and I grow lots of different varieties from seed (Viallii, Candelabra, Florida, Millers crimson etc etc)