Is it absolutely necessary? Perhaps not, except that this power is currently available to judges and juries. A judge hearing a case who considers that a visit to the premises is necessary to understand the situation can order such a visit. The same holds true when it comes to a judge and jury.
So, there may be cases where, indeed, the commission would benefit from viewing the physical premises. One example is a situation that occurred in Quebec some time ago, when a car fell into a lock. The question was whether it was an accident or murder. Visiting the scene and seeing the physical configuration of the accident often makes it easier to assess the circumstances.
I also think that if judges, who have decision-making power, have the right to go there, it seems rather odd that the commission, which has investigative powers, should be denied the same opportunity.