One of the biggest problems newbies in Starcraft have is when they are “floating money.” This is also referred to as having “Facebook money.” You only get to 3000 minerals and 2500 gas if you're not spending it, and when you're not investing your money in SOMETHING (production buildings, units, supply, upgrades, whatever!), it means you're falling behind. Having 3000 minerals doesn't win you matches, but if you turn that into 60 marines...
Real life is like this too! Piling up money in the bank is money you're not investing! Having some cash is good, but by converting that money into tangible assets you can make it useful.
In any macro Starcraft game (and Real Life is all about Macro), you need to expand your base in order to increase your income and survive. But if you expand at the wrong time, it's easy to get crushed. The timing of your expansion is based on a combination of how far into the game you are, your position relative to your opponents, and a few other factors.
Real life is just like this! If you don't expand into new projects and into new income streams, you risk getting contained and slowing your growth. But if you expand when you have too much going on, you won't be able to keep up and you will end up in a weaker position than when you started.