Why we use the trace calculation?

[Hluf]

Dear all
When we calculating the cross section of particles, we apply the trace calculation. This is not clear for me, why we use the trace calculation?
Thank you all.

 

[dauto]

The trace averages over many possible initial states. Useful when the initial state is not known or the beam of particles contains a mixture of states.

 

[dauto]

Also, if you're talking about intermediary (virtual) particles than taking the trace ensures that all possible intermediary states are taken into account.