Some specific strains help specific conditions (like some antibiotic-associated diarrhea). Benefits for healthy people in general are not established.
Strain-specific
Results for one strain don't apply to another — like dog breeds.
Evidence & sources
Probiotics may reduce the risk of antibiotic-associated diarrhea, but benefits for healthy people are not established. Evidence: moderate. Known: NCCIH review of evidence. Unknown: Which strains and doses help which people. Source: NIH National Center for Complementary and Integrative Health, Probiotics: What You Need To Know (Current revision).