SonarBells can be deployed for an indefinite period. This is because, unlike most transponders, they do not need battery changes and/or maintenance.
This is obviously a major operational/cost benefit in all situations. Particularly when deployed at depths or in hostile sea conditions and remote locations.
SonarBells can be designed to produce return echoes at specific target strengths. This allows them to be used as a reliable test target for sonars post manufacture or in service including in theatre for military operations.
There are many different applications in this regard, the most obvious being mine mimicking.