It is important that your wheels spokes are all evenly tensioned to help prevent them vibrating loose. As the spoke nipples vibrate loose, the spoke tensions become uneven. So the load is no longer being distributed across multiple spokes, and now being focused on one or two - Leading to spokes breaking.
Remember, the job of the spokes are to hold the hub in the centre of the rim, not to pull the rim straight when it bends. Continuing to ride on a wheel with broken spokes can lead to the rim distorting.