Clostridium botulinum is the bacterium that causes botulism (a form of food poisioning). The bacterium produces a potent toxin that causes muscle paralysis by reducing the release of the neurotransmitter acetylcholine from motor neurons.  For many other bacteria, the value (from the point of view of the microbe) is clear (for example, cholera is spread by contamination of food and water with feces from infected individuals; the cholera bacterium produces a toxin that leads to massive watery diarrhea which increases the likelihood of contamination), but I don't see how this toxin helps Clostridium.