When you have multiple links installed, chances are you want to control how your outbound traffic would utilize the links. For example, you want important applications (VPN, VoIP, etc) on WAN1, and the bulk traffic (HTTP, FTP alike) on others.
Make sure you are buying a device that has good traffic control capability. "Weighted Load Distribution" and "Session Persistence" are common algorithms; while "Priority" and "Enforced" are really thoughtful ideas. We will elaborate these with use cases in future.