Why are streptococcus and lactobacillus so important to humans?

They really aren't: Strep bacteria often hurt us, causing heart infections, rheumatic fever, tooth decay, and soft tissue infections. If you get involved with cults about stool microbes, they'll tell you that you need to eat special foods loaded with these microbes or your normal body flora and overall health will be ruined. This isn't my focus, but most of these claims seem overstated to me.