James Wallner at Legislative Procedure:
There are several reasons why senators’ procedural independence declined beginning in the 1930. For example, New Deal legislation packed the Senate’s agenda and its members were spending less time on the floor beginning in this period. And the decline in senators’ familiarity with the rules has persisted to the present day.
As a result of that decline, the Senate’s parliamentarian became central to ensuring the orderly flow of legislation through the chamber. For example, former parliamentarian Floyd Riddick noted in the 1970s that the parliamentarian’s primary responsibility is to advise the presiding officer “on every procedure that he must rule on or everything he should say even.” In this capacity, senators understood parliamentarians to – in essence – preside over the chamber. When the presiding officer is prompted to respond to a senator’s parliamentary inquiry or make a ruling on a senator’s point of order, the parliamentarian presently advises him or her of the appropriate procedure. According to Riddick, the parliamentarian “tries to keep the Chair posted on each step of the procedure before it arrives, if he can stay ahead; or if it’s too complex and he can’t be ahead, sometimes he has to whisper one sentence at a time to be sure that the Chair states what the procedure is.”
The Senate’s customary practice of rotating junior senators in the majority party to preside temporarily over the chamber encourages their dependence on the parliamentarian. These senators are likely to be unfamiliar with the Senate’s rules and, therefore, likely to rely on the parliamentarian for advice on what to say when one of their colleagues asks a parliamentary inquiry or makes a point of order.