The advantage of taking probiotics is to rebuild healthy, normal gut flora that's killed off by broad spectrum antibiotics, poor diet, and other lifestyle factors. When the normal gut flora aren't present, bad gut bugs are allowed to overgrow, creating all kinds of health problems - leaky gut, food sensitivities and more.
Probiotics aren't meant to help you gain or lose weight, but restoring balance to the gut should normalize your weight. In other words, you would gain if you're underweight and lose if you're overweight.
Here's an article you might find helpful:
Diagnosed with ulcerative colitis spring 1999.
Maintenance dose sulfasalazine.
Probiotics, l-glutamine and fish oil caps. George's aloe vera juice. Oregano oil antibiotic, antiviral, antifungal. Long-term remission with only minor blips.