why we need SSL Bridging ?

From an F5 perspective, it allows load balancing decisions to be made based on the encrypted data, traffic can be modified based on iRules, and, as Dylan stated above, security is very important to many major companies storing sensitive data. Plus, with the heavy reliance on cloud devices and external services, its quite possible to have to load balance to servers that require a public connection to get to. Unless you want to send plain text out to Amazon's cloud service, SSL bridging is your best bet.