PIP assessment optional tests

I recently had a PIP face to face assessment with Capita.

The assessor explained that there were 2 optional assessments at the end of the meeting, and if I declined to take part, it would not be held against me.

The optional tests were :-
1. Eyesight. Similar to an optician test of reading a board of letters, with each line lower being of diminishing height.
2. Physio movements. A series of very easy and undemanding movements.

I consented to the test, but am wondering if these are truly optional, what is their purpose ?