The easiest S/M example I can think of:
Any writer has more ideas than they can possibly pursue. Sit down and scribble in their journal for fifteen minutes, and most writers can easily come up with ideas for several years worth of work. Not hours, not months, *years.*
Yet in order to actually finish a nontrivial writing project, you have to spend large chunks of time focused on that project alone. Five or ten minutes here and there just isn't going to do it. YMMV, but I like to work in one hour minimum chunks, and two hours is better. There's also overhead associated with starting and stopping, switching projects, and so forth.
So, if I have ideas for 10 major writing projects, and I try to work on all 10 of them at once (in amongst everything else I'm trying to do), none of them will ever get done. To quantify this a bit, let's say that a major project is 50,000 words, and that including research and editing I have a net output of about 100 words per hour. That's 500 hours for one project, or 5,000 hours for all ten (sequentially). Given 20 writing hours per week (which is actually optimistic), I can finish one project in 25 weeks, or all 10 in just under five years.
But suppose I don't work on them sequentially, but try to move all of them forward at once. From experience, that will probably cut my net words per hour in half, doubling the total time required. I finish all 10 of them at once, but it takes ten years to do it. The last nine of which I spend in a homeless shelter because my clients got tired of waiting for me to finish things and quit paying me.
That's an especially dramatic example, but we all face the same tradeoff to some degree. Given finite resources, we can't do everything at once. the S/M list is for the things we choose not to do, but don't want to forget.
Katherine