The day of rest. Observing it grants time to perform miscellaneous tasks. It allows us to optimize our non-Sabbath time. It allows us to destress. This time is made available in order to course-correct our lives. To breathe, and to reflect upon the road ahead. Those who praise an extreme work ethic fail to notice the importance of this self-correcting aspect of rest. Reasons that - to me - justify keeping the Sabbath holy.
To keep the Sabbath holy doesn't mean to enforce a certain weekly rest day upon all of society. Whether it is Sunday or Wednesday does not matter. I should allocate at least one day per week unoccupied by any tasks. Completely free of large classes of obligations. A day of play. A day of rest. A day of whatever I make of it without worry.
Upon reflection, I have spent many Sabbaths learning. I have spent many Sabbaths improving inefficiencies in my daily life. A life of ceaseless learning only becomes better in the long term.
No religious associations are important here, the Sabbath is just a convenient name. The name gives the idea of a rest day historical weight. We can only conjecture why it was included into some religions; did it confer an evolutionary advantage to those religions? Whatever the cause is ultimately does not matter. I think it is important to have a rest day. Whatever religious traditions say about it can be interpreted as metaphor (GodIsAMetaphor).