Has your site experienced a large spike in traffic or an outage? Has spam increased? Has network performance dropped?
Bots flood the bandwidth of your website, making it slower or unavailable to legitimate users.
Perpetrators, including competitors, use DoS attacks to disrupt a site or take it down. Revenge, blackmail, disruption, and activism can motivate these attacks.
They can use multiple sources, which makes it impossible to stop an attack by blocking a single IP address.