There may be n reasons for tracing an UE. I have very limited information on this topic. Trace procedure consists of two sub actions. First starts the tracing for an UE and then collect and send Trace logs to TCE i.e. Trace collection entity.
As per my knowledge, tracing for an UE may be applicable for two purposes 1. MDT i.e. Minimization of drive test and 2. UE specific analysis (monitoring UE specific activity in network) w.r.t message exchanges over a number of interface like Uu, S1, X2 etc.