The advantages of microbes vis-à-vis destructive “hardware” such as guns and bombs is that they are often more effective at causing widespread damage without the need for explicit targeting. For example, a biological agent like anthrax can be spread through air or water supply with little difficulty, thus allowing it to potentially inflict serious amounts of harm on large numbers of people in a short period of time.
However, this also means that their use comes with a greater risk of accidental exposure – either to those who intend to deploy them or unintended victims who may come into contact with the microbe. In addition, depending on the type and strain being used, some microbes may not be able to survive in certain environments (e.g. extreme temperatures) which makes them impractical for certain situations. Therefore while there are definite benefits to using microbes over hardware explosives, there are also potential drawbacks which must be taken into consideration when deciding whether or not they should be employed.