Many feeds include it already, even when not advertised on the bag.
I find them effective at helping WHEN things are out of balance and
useless when they're not. The so called "bad" bacteria are there to serve their role, and life does not exist (in all it's possible forms) without them. This paragraph specifically being discussed with both my exotics veterinarian and also a human physician (PHD, MD).. knowing that opinions vary even among some of them.. these are how it's been represented to and understood by me.
Example of usefulness.. Some prebiotics or probiotics may help restore gut flora after rounds of anti biotics.. extremely important for hind gut fermenters like guinea pigs, rabbits, etc. Usually short term. Obviously there may other circumstance like under chemo therapy, etc.. but we're talking about chickens here. Could they be beneficial during treatment for coccidiosis and withOUT any added vitamins.. perhaps indeed! On the tail end of this, expiration date seems to be a factor with regards to keeping things on hand for when needed and shipping does not take place to my residence for at least 4 days but usually more. I haven't checked expiration dates in store.. but as a lesser sold item, my expectations are low.
I do find some of the
multi species ones to offer more benefit than others and the
seemingly "best" one (I have found) offers the inclusion of 7 strains of beneficial bacteria.. or whatever the correct terminology is.
Who benefits most is the company (including yogurt companies) selling them to everyone who believes they will extend their life and somehow avoid the inevitable process we all will face.. some call it death, transition, crossing over, etc.. sorry! I ate it when I thought it was good for me. Now, I realize life is too short to eat things I don't enjoy because "someone" says it's "good" for me. Why preserve a prison called safety and stay trapped like an inmate if my mind is the jailer!
One of the ways folks like to "add probiotics" and an example of what I consider not useful
long term is by fermenting their chicken feed.. which I did exclusively for 2 full years for a flock of about 82.. Truthfully, genetics plus husbandry and nature play big roles.. we like to think we have control or influence.. but what we're having is an experience and life is our greatest teacher. I'm for real SIs!

(sorry if that's a misuse of your username, it's meant as a clever fun poke.)
Please don't confuse poop texture fully with something being needed or off.. Every 7-10th (approximately) will be cecal poop and also the more water they drink the runnier the droppings are since all urates (urine/pee) is passed with the poo.. so in other words sometimes runnier in hotter weather.. Poop watching can be fun, lol!
I also however "believe"in the placebo effect and have witnessed it to be as effective as *most* any treatment.. So if what you're using (such as probiotic for animal it's not native to) works for you.. then Right on, Right on!