Your units are money
Once you get the meaning of latent credits, repairing gets a much more significant meaning. A soviet Crusher Crane for 1500 credits has already payed off when it was able to repair two Twinblades worth 1200 credits each on red health!
The more a unit costs the more you have to benefit from it before it dies. A Flak-Trooper costs 133% the price of a Javelin. That means if you decide to train one, you better make the best out of it! A Javelin is incomparably more powerful than a Flak-Trooper so it's going to be fairly hard to use a Flak-Trooper 1,33 times as effectively than your opponent uses his Javelin. That's the reason why Flak-Troopers are primarily used for drops (and other special purposes) - their Mines are much more powerful than their main weapon.
When RTS emerged, strategy games received a new resource: Time. As we apply the definition of resources to time we realise that time can be gathered, spent and consequently wasted as well. Time is the reason Empire players build cores in the order Generator-Dojo-Dojo-Dojo instead of Dojo-Dojo-Dojo-Generator. Time is a factor that always plays a role with build orders. You waste time when you don't execute your build order properly. In that case you start collecting resources later which equals to a loss of credits.
It even plays a role at unit orders. For example the question - should soviets train the Engineer first or a Bear? You should always train the scout first! If you do so, the Engi will only be two seconds late which is 15 gathered credits from the derrick. 15 credits is not at all very much. Losing 15 credits but in return knowing whether your enemy is doing a fast Airfield build order ten seconds earlier is definitely paying off.
Obviously, if you are in a situation where you need a unit as soon as possible, it will be best if you get it as quickly as possible. You shouldn't waste your time training a unit that is less effective first.
Time saves lives. Your life in particular. Let's look at the following scenario:
You attacked and you lost everything in the fight, because the police called you and asked if you ever downloaded any copyrighted material, so you couldn't micro. Let's say, your opponent has 5 units left and you lost everything. In this example you and your enemy reinforce equally fast. After ten seconds both of you produced one unit. Now your opponent has 6 times the amount of units you have. Now let's see what happens as time passes. After another ten seconds you have two units vs. your opponent 7; now he has only 3.5 times the amount of units you have. After another ten seconds it's 8-3, that's 2.67 times as many units; later it's 2.25, then 2.00 - and so on.