It is not necessary to have a Store to sell on eBay. If you have fewer than 54 unique items it is more expensive to sell through a Store than just to list.
With a Store you are not eligible for the 50 Free Auctions or for most of the free listing promotions that eBay runs constantly.
Stores are useful for sellers, like me, with hundreds of unique and slow selling items. And for items that need to be sorted because the seller has so many. My 1500 odd books are further sorted in my Store into 20 categories. so readers looking for science fiction don't have to wade through cookbook titles.