The addition of new physical (real) servers, and the removal or failure of existing servers, can occur at any time, transparently, without affecting the availability of the virtual server. The slow start capability of IOS SLB allows a new server to increase its load gradually, preventing failures caused by assigning the server too many new connections too quickly.
IOS SLB supports fragmented packets and packets with IP options, buffering your servers from client or network vagaries that are beyond your control.