Taking a night photograph

Text: an original contemporary personal account-style practice extract.

Before taking my night photograph, I rested the camera on a low wall because I had not brought a tripod. I used the self-timer so that pressing the button would not move it during the exposure. A cyclist crossed the frame, leaving a blurred streak of light. I kept that photograph as well as the empty-street version I took afterwards. The blur was not the picture I had planned, but it recorded an interruption that the clean second image could not show.

From the extract, identify one reason the writer used the self-timer.