2 of 2 added tags to indicate what the question is about

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

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?