The reason to use a BOS for a Web App are pretty much the same as the the reason for using a BOS for a Windows App. With a small number of clients, the BOS may not be necessary. However, as the number of clients increase, you will discover that at some point, you will need a BOS to get adequate performance. In particular, the BOS will give you much better database performance because of connection pooling..
We have a client in the UK who is getting good Web App performance with a small number of clients, but with a large number of clients, performance degraded. The customer elected to go with a BOS.