You can grow kale all year round..but I suppose lot of people do grow it for winter/spring use maily as there is so many other bassicas available that are bit more..hmm, not sure what word to use, well..more liked than kale...

You can sow kale 'thickly' and use 'baby' leaves as tender salad greens..some kale varieties are added into commercial salad mixtures and its quite possible you've eaten them without knowing it.
Or..you can grow then thinly into individual robust plants..those I sow late spring/early summer..and sometimes later too

..to be cropped later on the year or during winter.
I usually start cropping from leaves lower down so I don't disturb growing point..I cut out the stalk and mid-rib of the leaf and use the 'soft' part...thats when they are mature leaves as the the middle 'vein' don't cook soft without 'killing' the rest of the edible bits.
In spring plants start to make loads of new lush growth before they flower..those leaves I use whole and I tend to pick the quite small..just trimming the stalk off.