Disclaimer - I've never played Starcraft or Magic: The Gathering.
But I expect that in both of them, just like in real life, there is some value to having unspent liquid assets, in that they give you optionality. Unspent liquid assets can be converted at a later time into marines/computing hardware/other 'real' assets, depending on what is most needed at the time. If you turn all of your liquid assets into illiquid assets as soon as you get them, you lose that optionality.
You know, I hear what you're saying, but I don't think I've ever heard that.
Usually better players will give newer players a simple heuristic like "don't let your mineral count go over 1000", which seems generous even. Something more hard core would be like 800.
The point is, there aren't many scenarios where "saving" your money would be a good thing. Except for "expensive" purchases, which usually hover around 400 and no higher.
Units and buildings (things you spend money on) in StarCraft also cost time. So if you wait until the actual time you need things to request their construction, it's already too late. So there's some forecasting going on so that you can soak up the construction time.
Also, your economy is a forever investment. If you aren't attacking, you're building/expanding. So there's essentially never an ideal moment, there's always a fall through case. You're always spending your money on something.
The one possible exception to this would be the Zerg, who play in a more reactive style. Which means that their per unit construction time is on average less than the other races (as I'm saying this, I'm actually just guessing that, watch me be wrong). The point is, the Zerg are more reactive than they are forecasting, so maybe in this scenario, some amount of saving would be apropos. But still, you are spending on your economy until the last possible moment and then transitioning into an army. Zerg happen to specialize in "on the field" adaptation and parallel construction, making their throughput higher for shorter amounts of time.
But it could be argued that none of that money was "optional" or idle. I knew that I was saving for an army and as I was scouting I knew I was saving for a specific kind of army. Maybe burn 2/3 on unit construction and save that last 1/3 to use their on the field adapt ability (like baneling morph).
The problem is that Starcraft is a bad model, because in Starcraft, all the options are known in advance. In the real world, not so. Someone can invent something new tomorrow and all of sudden you may find those cash handy.
Also, 2-player Starcraft is an antagonistic game. In the real world, there are coalitions, and the game is more complicated. Having cash on hand can give you an edge when the coalitions change.
But I expect that in both of them, just like in real life, there is some value to having unspent liquid assets, in that they give you optionality. Unspent liquid assets can be converted at a later time into marines/computing hardware/other 'real' assets, depending on what is most needed at the time. If you turn all of your liquid assets into illiquid assets as soon as you get them, you lose that optionality.