DDoS protection uses algorithms and advanced software to monitor incoming traffic to the website. Any traffic that isn't legitimate is denied access, whereas legitimate traffic continues to filter through to the site.
DDOS protection is necessary if you want your website to not become inaccessible using botnets. It also prevents your server from being overloaded and exhausting its Bandwidth. It also ensures that your mitigation solution works during the attack and that the threat is prevented in the cloud before it reaches the origin.