In the days before television and computers, people actually read books and wrote letters on paper with pens and typewriters. They regularly read and wrote words, and this constant repetition helped them to remember how to spell the words correctly.
But in those ancient days, they still memorized lists of words. And they did it the way I did, and my father did, and you most probably did. They received twenty words on Monday and had a spelling test on Friday. During the week, the teacher helped the class study the words, but the onus fell to the student to memorize them for the Friday test.
I don't see anything wrong with this system. I used it myself when I taught school. Parents expect this method which gives students a performance record to show their parents. The main problem is choosing appropriate words for the students. Depending on their ages and abilities, I always preferred to use words that illustrated a spelling rule.
Taking words from lists like “The 50 Most Difficult Words” or “The 100 Hardest Words to Spell” is, of course, a silly waste of time. It promotes short-term memory only. The ideal would be for students to read more books or magazines or even comics. In the meantime, we have to fall back on old-fashioned memorization. It worked for our parents. Why shouldn’t it work for us, too?