Sometimes you don't want any traffic from a certain organization... none at all, not even from their employees. An effective way to do just that is to determine every IP adress that belongs to a certain Autonomous System which is how IPs are generally allocated to organizations that have a significant1 presence on the internet.
Footnotes
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Signifigant in this situation means organizations large enough to want/need a huge block of IPs. They're usually assigned either to organizations that have been on the internet for a long time, or to organizations that deal with big internet infrastrucutre. ↩