The immune system is highly complex in of itself and there is still lot of discoveries being made with conducting research in this field. The reason why B cells/T cells need more than one binding system for activation body response is due to it being a third check point the immune system uses for immune cell activation. Without going into much more advanced detail (there are specific courses for this) the simplest answer is as follows:
Taking B cells as an example, who function is to make antibodies (plasma cells) for the activation of T-cells, requires the concept of the "two-hit" hypothesis in which the cell is "turned on" but not activated with binding to one specific signal but needs a second signal to enhance its function and be turned on full and actually undergo its activation through stimulation of t-cell dependent activation or T-cell independent activation. In T-cells the "Two hit" hypothesis remains true for activation but in reality actually requires three separate signals to become activated undergo its immunological function. If any on these cells are missing a signal for activation the cell just remains "on" but not functionally activated and will eventually undergo cellular anergy and die through apoptosis. This is critical for cells traveling throughout our lymph and circulatory system don't become activated through recognition of our self receptors and mount an immune response against ourself. If this were to happen (say with T-cell development in the thymus) they are automatically signaled to go through apoptosis.