Cost of delay. If you have two projects which when finished will generate (say) the same amount of value per time and it takes 2 weeks to do the first and 1 week to do the second, by sequencing them in the other order you gain 1 weeks' worth of value. If instead you interleave them, working "in parallel", with no real gains in total time, you lose 1 weeks' worth of value. Also and more obviously, if one project generates way more value per time, finish it first. If a project loses value while not finished, consider finishing it first. (for example taking an idea to market - your market research undergoes depreciation)