I think it's OK (though others may not concur) because this will give us a chance to discuss grow bags sold in U.S. vs. the ones that have been in use in U.K. for some time now.
So far, the "grow bags" that I've seen in U.S. catalogs are no more than empty containers of various sizes and shapes made of sythetic fabric-like material.
The ones in U.K., as JONA described come filled with soil mixes that are, presumably, properly blended with the soil texture and fertilizer for the type of veg it is labeled for (as in this case, Tomato and Potato). I believe you're supposed to situate the bag, cut a small opening, moisten the mix, settle the bag, then cut openings for planting, and plant. The product concept "grew out of" the trick some gardeners were advocating for simply making openings in bags of "compost" or "potting/soil mix" (as we call them) for planting and for drainage. Since the plastic bags they normally come in is not always sturdy and deteriorate in the sun (and face it, not attractive... oh and insufficient depth for larger veg -- at one point, people were stacking bags and cutting through to the lower bag(s)), clever folks came up with the idea of packaging specialized potting/soil mix in sturdier, special-sized bags. Perhaps also with intent for post-consumer market of sufficient additional potting/soil mix to be purchased for re-filling the bags (are they re-usable?).
With U.S. ones, you will need to buy the potting mix separately. Sufficient to fill the bags. In catalogs, they usually sell a specialized "container mix" alongside the grow bags that is well-draining and is pre-mixed with water-gel polymer for moisture retention. I don't think the U.S. market is sophisticated to the point of making specialized individual veg soils except maybe tomato, though I think I have seen something labeled "for vegetable gardens". (aside from the houseplant market -- as in african violet, cactus, orchid, citrus, etc.)