Statecharts are highly useful to represent behaviour. Sequence diagrams do not capture it as much.
Timed behaviour, like timeouts can be represented in statecharts by having transitions on time based conditions. For example an event puts the system in a 'waiting' state, and in the waiting state there is a 30 second transition to a 'fail' state unless some other event happens which pulls the system out of the 'waiting' state.
Also it provides a good indication of what behaviour is valid and what not valid and what are don't cares.
External interactions of a system can be model as the state changes of a system.
They also have 'memory', special elements that remember which substate the system was in before it jumped out of the state last time.
I recommend David Harel's very interesting paper on modelling behaviour with state charts.
Timed behaviour, like timeouts can be represented in statecharts by having transitions on time based conditions. For example an event puts the system in a 'waiting' state, and in the waiting state there is a 30 second transition to a 'fail' state unless some other event happens which pulls the system out of the 'waiting' state.
Also it provides a good indication of what behaviour is valid and what not valid and what are don't cares.
External interactions of a system can be model as the state changes of a system.
They also have 'memory', special elements that remember which substate the system was in before it jumped out of the state last time.
I recommend David Harel's very interesting paper on modelling behaviour with state charts.