The economic principle of scarcity explains the dilemma of matching scarce resources against unlimited wants. Customers, clients, projects managers, etc., typically provide the unlimited wants. If I'm honest about it, I'm just as guilty about MY unlimited wants.: tuning another report, scoping a a new feature, learning a new technology, working to solve another problem, all seem to compete for time.
So, how do I determine the trade-offs? Entire books have been written and self-help gurus burn up the miles attempting to answer just that question. For me personally, though, I usually end up focusing on what the late Steven R. Covey called "Quadrant 1." These are urgent and important items, and they typically have to be done now. After that, I prefer to work on items that are important but not necessarily urgent. Interestingly, my work satisfaction increases when I have worked through my Quadrant 1 items first.