Your users' experience in terms of navigating your site should be a great reason. Users have many options when it comes to consuming content on the internet, and a slow performing website will frustrate your users, sending them elsewhere. A faster and more responsive website will keep them more engaged and focused on your content, rather than waiting for scripts and images to load.
Another reason is for SEO. Google has announced they are using page speed in their ranking algorithm.
This means that faster performing sites may rank higher in search engine results. As developers and marketers look to optimize their sites, page speed should be among the top optimizations to consider.