Is it to create a friendly bacterial atmosphere for your individual GI tract bacteria to grow?
Exactly. They crowd out the bad guys and stimulate your own bacteria to grow. Take also some prebiotic fibers (AFTER! some days//weeks of probiotics... let the probiotics clear a bit the gut first).
Florastor is a beneficial yeast (Saccharomyces boulardii lyo) and only remains for about 4 days after discontinuation. Other strains of good bacteria have proben they stay in your gut about 15 days after discontinuation (lactobacillus gg, lactobacillus plantarum V-299...)
This is something you must care about when you feel die off. You can think that taking 5 or 10billions/day is not too much but they are ¿acumulating? (excuse my poor english) in your gut so after some days you will have lot of billions.