When you find seeds in a bag of cannabis that should really be sensi, the seeds are almost always the result of the plant selfing. This means that the female plant sprouted male "parts" that released viable pollen. As the resulting seeds have no xy chromosome, they are females, which may sound good, but it isn't and this is why. The resulting seeds now carry the hermie genetics and have a very great chance of producing male parts and making seeds themselves. If there are only a couple of seeds, it is not a disaster. But sometimes it is not a small amount of seeds, Sometimes your entire crop is pollinated and every plant will be loaded with seeds. Up to 75% of your bud weight can be in seeds. Not only is seeded bud a huge PITA to clean and smoke, but the resulting bud is not as potent because the majority of the plants energy has gone into creating seeds and not THC.
As quality seeds with known genetics are so available and so reasonable, I do not understanding anyone taking this huge risk with their crops. It takes about 4 months to take a plant from seed to smoke. It just sucks to get 3-1/2 months through and lose virtually your entire crop to seeds. There is also the chance that the plant was accidentally pollinated with stray pollen from ditch weed, which would also suck.
There is a huge advantage in picking out a strain that YOU like and that has the characteristics you are looking for. In addition, you can almost always find someone who has grown the same strain and can give you growing advice. The breeder's info will tell you things like how big the plant is expected to get, whether it grows short and bushy or tall and lanky, how long it takes to flower, any other peculiarities the strain may have, etc, etc.
I ordered the Exodus Cheese as it is supposed to be good on pain. It is wonderful to be able to decide what you want your cannabis to do for you and then go look for that strain. I find this site to be a good place to get strain info.
https://www.leafly.com/start-exploring