Actually creating an add-on allowed a lot more than offering a simple web-site. Remember Feedly is not new, it's been out there for years. Back then, offering an add-on allowed them to lower the stress on their platform by allowing most of the network trafic to be directly between your browser and google's servers. Now they have migrated on their own Cloud platform, they will probably very soon offer a plain site version without the need for add-ons.