What is the point of asking the Prime Minister about their engagements for the rest of the day?

Question

Every week (or so it seems) the Prime Minister is asked the same meaningless question and gives the same meaningless answer. For example:

Huw Merriman (Bexhill and Battle) (Con)

Q1. If she will list her official engagements for Wednesday 15 March.

The Prime Minister (Mrs Theresa May)

I am sure that Members will want to join me in wishing people across the UK and around the world a happy St Patrick’s day this coming Friday. This morning, I had meetings with ministerial colleagues and others. In addition to my duties in this House, I shall have further such meetings later today.
_(Hansard).

It seems that it is a tradition to ask the Prime Minister what his/her engagements are for the rest of the day at the start of each Prime Minister's Questions. The MP who asks the question gets to follow up with another, more substantial question.

The Prime Minister answers questions from MPs in the Commons every sitting Wednesday from 12pm to 12.30pm.

The session normally starts with a routine question from an MP about the Prime Minister's engagements. This is known as an 'open question' and means that the MP can then ask a supplementary question on any subject.
_{(UK Parliament Website).}

Is there an actual purpose behind this procedure or is it (as it seems to be, for me) a meaningless ritual which is just done for reasons of tradition and posterity? I've certainly never seen the Prime Minister actually detail their real engagements anyway. They always respond with the same scripted reply.

Is there, or has there ever been, a purpose behind asking the Prime Minister what engagements they have for the rest of the day?

Answer 1

Questions in the House are required to be relevant to the Minister / Department answering them, in the PMs case this is simultaneously everything and nothing! Virtually every policy area is covered by another department or minister yet the PM has overall responsibility for all.

MPs also want to retain the ability to ask a topical question (written questions must be submitted 3 days in advance) so the current practice has evolved - the PM will be asked what their engagements for the day are (clearly on topic for the PM), the PM responds with "various meetings and discussions" and then the real question is asked as a supplementary. The supplementary should be related to the main question so normally it is asked in a fashion such as "during the day will the PM take a moment to think about XYZ".

Link to official description

EDIT: As has been pointed out the Leader of the Opposition (Currently Sir Keir Starmer of the Labour Party), and the leader of the third party (currently Ian Blackford of the SNP) have the automatic right to ask questions.

Traditionally other backbenchers when asked to speak will say the number on the order paper "no 7" and then the PM will respond "I refer to my previous answer" and the the backbencher will ask their supplementary question. However it seems this practice may have been done away with see for example this recent hansard record or this BBC iplayer video.

Answer 2

The first question is given to the PM in advance, the second isn't. The second question is meant to be in reply to the answer to the first, but isn't always.

By asking a bland first question to the PM, the Shadow PM can ask an awkward question about any current issue as the second question, which the PM has to answer off-the-cuff. The idea is to trip the PM up.

Answer 3

I'm sure I read somewhere that the reason to ask the Prime Minister about engagements for the day is because that forces the Prime Minister to come to the House to answer the question. If you ask about Health Policy, the Prime Minster can refer the question to the Health Secretary. Money questions to the Chancellor etc. Only by asking a question that is specifically about the Prime Minster, who has no portfolio, is the PM forced to come the House to answer.