Sometimes there isn't enough money or quite enough classes for, say, 4 full time teachers in a department so the school recruits 3 and uses supply to cover the shortfall.
Sickness rates in most organisations are around 5%. I suspect it's a bit higher for teachers as if you are feeling off colour in some jobs you can take things a bit easier but in teaching you have to be up on your feet in front of 30 lively students all day so they choose not to struggle in. That means in a typical week of 30 lessons your daughter will have a couple of lessons covered by supply teachers.
Also, for some subjects in some parts of the country, recruitment can be a bit more difficult so supply teachers are used.