Lecture/seminar/tutorial slots change every term, so three times per year and that's the official timetable/schedule, which is often changed during the term and there are additional "one offs" too such as special presentations/lectures, career sessions, etc.
Zero hour contracts are often used to cover absent "permanent" staff taking holiday days, sick days, a few hours for a GP or hospital appointment, and all manner of other "unplanned" time off, hence why they're often called in at short notice, usually to cover someone who hasn't turned up or called in sick at the start of their shifts.
The "low hours" to avoid NIC is no longer valid really since Rachel lowered the NIC threshold which catches a lot more "low earners", i.e. part timers on few hours.
Basically the more "workers" that an employer has on speed dial gives more options to cover when other staff aren't in for a multitude of reasons.
It's easy to say that, say, a small shop should just have 3 staff and one of the others should cover when employee A is on holiday or calls in sick, but if the other two aren't available then the shop is stuck. Even more so when the other staff don't want to work more hours because it affects their benefits, i.e. UC or tax credits, etc., which is also a big problem - lots of part timers plan their working hours very carefully to avoid losing benefits so won't be willing to "cover" for an absent colleague if it means breaching whatever threshold they're staying under.