They left the sides open to be able to put a foot under to go easier on your lower back. I would either close it in or put something on the ground to block grass/weeds. Tough spot even for a string trimmer.
https://www.noble.org/globalassets/docs/ag/pubs/horticulture/nf-ho-15-01.pdf
Is there anything more useful on the farm than junk tires? I think not!
Great set-up. Our plant is to ‘raise’ all of our raised beds as I get older by adding a second ‘layer’ of wooden frames to them.