A large share of what a crew shoots never appears in the finished film, and that is the intended outcome rather than waste. Coverage is bought as insurance against decisions that cannot be made on set.
The edit is where the film is actually assembled
What plays on set and what plays in a cut are different judgements, because rhythm, comparison and context only exist once scenes sit next to each other.
An editor needs alternative angles to shorten a scene, to remove a weak line or to change where the audience is looking at a given moment.
Without those options, the scene can only be played as shot, and a problem discovered in the edit has no available solution.
Performance is uneven within a take
A take rarely fails outright; more often part of it is strong and part is not, and the strong portions are distributed differently across takes.
Cutting between angles allows the best sections to be assembled into a performance that was never delivered in one pass.
This is only possible where matching angles exist, which is why a scene is played through repeatedly from several positions rather than shot once from each.
Technical faults are frequently invisible on set
Focus errors, reflections, boom shadows and background problems can survive a monitor check and become obvious on a large screen.
An alternative angle covering the same dialogue means a fault costs a cut rather than a reshoot, and the difference in cost is enormous.
Sound is the same case. Where a line is unusable from one position, coverage from another may have captured it cleanly.
Pace requires material that can be removed
Finished films are almost always shorter than their assembly, and tightening depends on having places to cut away from.
Cutaways, reactions and inserts exist largely for this: they allow time to be removed from a scene without the join being visible.
A scene shot as a single sustained take cannot be shortened at all, which is a deliberate commitment rather than an efficiency.
The discipline is knowing when to stop
Coverage costs time on set, and time is the resource everything else competes for, so shooting every angle for every scene starves the scenes that need attention.
Experienced directors cover selectively, protecting the moments where the edit is likely to need options and committing on the ones where it will not.
That judgement is what separates efficient shooting from both extremes: shooting everything and leaving the edit no way out.