The bridge / switch concept wasn't created as a security solution. Its main goal was to alleviate the problem that arises when there is a high number of devices attached to a single ethernet network.
When ethernet networks were a bunch of hubs together you could have hundreds of workstations sharing the same collision domain and the same broadcast domain.
As more stations you have in a single collision domain, the chances of transmitting without collisions is lower, and the results are: more retransmissions and a perception of slow network.
A bridge divides the network in a number of collision domains equal to the number of ports the bridge has and maintaining a single broadcast domain.
The result is less collisions for the same number of workstations, improving the general speed of the network.
The ultimate bridge is the switch, where each workstation is inside its own collision domain, letting the workstations to use full duplex (because as there are not collisions, the workstation can transmit and receive simultaneously).