Several reasons in no particular order:
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You never “have” to do anything, most of those guidelines are for people to use until they reach a point where they are no longer relying on guidelines. My workout initially began with things split into sets.
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Time. It cuts the overall time of my workout down significantly which is a major constraint for me.
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I’d like to avoid adding any muscle mass whatsoever to my arms. I could boost the weight but then I’d start building mass. I’m a relatively top heavy guy with pretty big shoulders and arms, it’s always been that way even when I was ultrarunning and not doing any upper body, it’s just my natural build, maybe I developed it on the farm, I don’t know, just me and all my brothers have it. But regardless building arm strenth is not a priority for me, I just want to work out enough to keep tone as opposed to flab since core is really most important for kayaking, the upper body weight just works against you rock climbing and I don’t like the meat head asthetic.
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I don’t view it as one set of each exercise but rather grouping the two front raises together and the lateral raises it’s one set of 3 variations of shoulder raises, two of pushups, then the one set of curls. I focus so much on maintaining those shoulders to prevent another shoulder tear from the stress you encounter from powerful water features in whitewater kayaking. My shoulder strengthe is solely responsible for limiting the damage in my first tear, these were part of my therapy, and they’ve been incorporated into my current workout.