Traditionally assignments handed in on paper need to be handed in at the same time due to time constraints on the instructor, to prevent plagiarism, to maintain pace since most of the teaching happens through physical lectures and if someone is behind they will miss out etc.
Since a massive chunk of grading is automated nowadays with online access to materials, not to mention practically all teaching being online for the time being, most of the reasons that make assignment deadlines necessary do not apply anymore. After all if someone demonstrates the same learning goals as the next person, what is the difference if they do it a couple weeks later? It's not like the instructor needs to spare extra special treatment time for the student doing it slower for grading or even lectures, it's all asynchronous anyway.
So what is the point of maintaining the practice? Some common arguments I see include "deadlines are part of life, students need to deal with it" or "course design requires so" which are not really arguments if you think about it. Another common one is preventing students from getting too collaborative, which also fails since time constraints do not apply anymore, given the resources and technology and whatnot - people can plagiarize entire job interviews spontaneously these days.
The only big argument that holds any water that I have seen is the issue of students suddenly hitting the end of the semester with massive amounts of work piled up and all the cramming and health issues and stress with that, so it is really for their benefit to impose hard deadlines to avoid that. Paternalism in this aside, that to me seems like more of a consequence of the persisting work philosophy of meeting smaller deadlines with whatever one can put together, rather than meeting learning objectives as completely as possible within a given semester. This in turn causes a great deal of common procrastination that it is offered as a solution for.
So what else am I missing that makes it actually beneficial to set traditional deadlines and penalties even for completely asynchronous classes?