Here are just a few points I’ve found listed; there are certainly many more….
Network neutrality avoids that ISPs charge online services such as XBox Live, Playstation Plus, Skype, and Netflix for “fast lanes”; they can make the services more expensive for internet users and also may prevent small companies from the capacity to compete with the big companies who have the budget to reach agreements with ISPs.
Net neutrality avoids discrimination among users ensuring similar access to information for people of different socio-economic status.
Network neutrality helps to promote freedom of choice, as ISPs cannot obstruct or incentivize particular contents or sites over others.
Anti-blocking and anti-discrimination rules prevent the capacity of ISPs to arbitrary decide to limit access or promote some type of content. The role of ISPs is to only “transport” data to the users that have paid for delivery, and therefore they should not shape content consumption patterns.
Net neutrality promotes a level playing field for competing companies.