Internal combustion engines use their own motion to power themselves. The rotation of the motor draws air and fuel into the combustion chamber, where it is compressed and ignited, causing more rotation and continuing the process. However, for this whole crazy process to work you need something to get it started in the first place. In most cases, this is actually a whole separate motor, so your engine is really two engines: the main engine and the starter engine.
It's worth thinking about this as far as other processes, because I see similar patterns in a lot of places. You have a self-supporting inertial system which, once it gets going, should sustain itself and be relatively robust. However, before it gets to that point you need to have a separate starter system to get that system working in the first place. And both systems are important! If your inertial system isn't good enough, you'll have to keep going back to the starter system. If the starter system isn't good enough, you'll never get to the inertial system in the first place.
Why not just have a starter system and no inertial system? Well, usually a starter system is unsustainable. For internal combustion engines, the electric starter will burn itself out if used for too long. They're also usually not very efficient because, well, why bother? The expected duty cycle of the inertial system is orders of magnitude higher than the starter system, so it makes more sense to focus your optimisation there.
And why not make an inertial system that doesn't need a starter? In some cases that's possible, but often the tradeoffs don't line up. It's much harder to make one system that can cover the entire range of conditions than to make one system optimised for the initial conditions and one optimised for the steady state once everything gets going. You might need to go back to trading with precious metals if society collapsed, but that doesn't make them a good candidate for everyday use now.
I think one area where this two-system analysis is particularly useful is when working on habit formation and other personal systems. Once you're doing something on a regular basis it's easy to keep doing it. Ideally, even easier than not doing it. But there are two ways that not understanding the starting vs inertial system can trip you up: firstly, just because it's easy once you're doing it doesn't mean it's easy to start. And the other way is that you can sometimes fall into a good inertial system by chance, without understanding the starter system that got you there.
Which is all well and good, except that what happens when something pushes you out of your inertial groove? Your daily running habit gets put on hold for a few weeks because you have a twisted ankle, or your streak of productivity gets halted through burnout or time off. That's when you need to turn back to your starter system to get things going again.
But if you never really had one in the first place, you might find yourself stuck wondering why your engine isn't moving when it was working fine a week ago.