There are certain activities that are good for you
*Getting enough sleep
And there are certain activities that increase your work productivity
*Planning your day/week/work
*Prioritizing your activity
*Investing in skills that increase your productivity
And each of these is very powerful.
However selecting habits that work well with each other can provide greater results, instead an additive situation where 3+3 = 6, you can multiplicative force 3x3=9
You can call these force multipliers, using these intelligently is wonderful because they are as close as you can get to a free lunch.
Getting to a deep, or mastery level of a single skill takes a huge amount of time. Your initial gains are fast as you pick up the basics but that progress gets harder and harder to make. Combining a skill with a force multiplier allows you to get around those diminishing returns.
In addition, we are all time limited and selecting adjacent skills or habits allows you to make the most of your time. Most people and companies are unwilling to pay for someone who is mediocre. However there is a high premium attached to being the best.
If you intelligently stack your skills you can get this multiplicative effect, which can turn someone from a mediocre performer in a bunch of areas into a leader in one, and frequently it is easier to get there than it is to be the very best at one thing.
Think about it this way.
You start to exercise, immediately start to get stronger, maybe you lose some weight. If that is all you do, eventually you will reach some sort of a plateau. However, say you start working out in Jan, and then in Feb after you have established your exercise habit, you start improving your diet. Your recovery quickens, you lose more weight, you get even stronger without hitting the exercise plateau. Take it one step further, in March you start to focus on improving your sleep, you rest better, you recover more quickly, lose even more weight. These are three habits, each of which have documented scientific evidence behind them for improving your health, helping you lose weight, and if you combine them you get an even greater bang for your buck.
Clearly all of your skills can be directly tied to each other. Maybe you want to be a scientist who plays guitar and surfs. You don't select skills simply because they work together. If you have an interest in an area, then take the time to build the complementary skills, the extra understanding you get and the force multiplier effects will often pay off more than going all out in one thing.