We use the passive voiceThe form of the transitive verb that shows whether the subject acts (active voice) or is acted upon (passive voice). when we do not know who performs the actionA general term expressing that somebody or something does something. or it is not important. In this case the agentThe doer of an action; the person or the thing that does something. E.g.: True Romance was written by Tarantino. ("Tarantino") is not mentioned in the passive sentence.

Tom’s bike has been stolen.
I was advised to obtain a visa in advance.

Passive voice: change of focus
Uses of passive voice Passive voice: generalised agent