I suspect that where universities are checking attendance they're doing it at least in part to protect themselves if students later turn around and claim the uni has failed them in some way.
Take the plagiarism lecture example. If a student plagiarises something in one of their essays, the uni can legitimately say "we told you, very clearly, what plagiarism is, what we expect from you, and what the consequences would be if you did plagiarise" and the student doesn't really have any comeback from that - otherwise, they may argue that they weren't clearly informed and didn't understand their responsibilities.
Equally, in terms of general attendance, if a student's attendance is poor and they then fail, they (or more likely their parents, who although their child is an adult may well still try to be heavily involved and who have in many cases made significant financial contributions) may turn around and say that the uni never indicated that there was a problem, and that they should have stepped in to prevent the student from failing in that way.
Equally, not attending lectures can be an indication of deeper problems. It may be that a student just can't be bothered, and to be honest, that's their choice. But it could also be a sign that the student is suffering from some significant problems - a developing mental illness, a medical condition, a major family issue, something like that - and, if so, the uni would be failing those students pretty badly if they never pick up on the fact that the student needed help and support because they just assumed their not turning up was due to a heavy night down the pub the night before, or whatever.