It's by no means a necessity anymore, but I think premade anki decks have utility for some learners.
One advantage of anki is that the workflow is extremely straightforward and requires less attention or executive function. It's like a pre-drawn vial of drug ready to be injected, while class lectures are more like chemical reagents that have to first be processed into drug, then drawn up, then injected. The practical usage of anki also inherently lends itself to operant conditioning and reward learning in a way that reading lecture notes doesn't - you're in this artificial environment encountering discrete structured tasks, in which you receive immediate feedback/reward, and can have momentary lapses in concentration between encounters. It feels good to get cards right and see the number of remaining reviews tick down for the day, in comparison with the relatively ambiguous and unstructured alternative of passively reading/listening to lectures.
Many of my friends were able to passively read/listen their way to good grades and step scores - for them, the only additional utility of anki would be those first aid tiny minutiae. For others, the gulf is much larger - either barely scraping by using passive techniques (still with significant time investment), or exceeding expectations using anki (with only marginally more time investment). Anki still does a very good job of teaching the basics, even with more conceptual things - cards can sort of be used as "mini practice questions" in which you try to synthesize what you know and make sure your answer to the card meshes into that conceptual framework. This usually comes with liberal use of the "extra" section and supplemental figures, notes, and tables.
It'd be interesting to do a hypothetical study examining preferences and learning outcomes with different learning styles (passive vs anki) and correlating it with a few key factors, both psychological (probably executive function, and maybe something like Big 5 neuroticism) and neurobiological (rate of dendritic spine formation and stabilization, long-term potentiation, long-term depression/extinction learning). Perhaps if you have relatively high executive function, low neuroticism, and learn things quickly (rapid spine formation/LTP, slow LTD) passive works well, and for the opposite (lower executive function and slower LTP) anki is better. For the first learner, anki would feel restrictive and unnecessary - being forced to learn in an overly structured manner for marginal gains. For the second learner, anki is freedom - freedom from the disruptions of anxiety or lapses in attention, and additional executive function required to drive home a slower more stubborn rate of learning reinforcement.