JIT - Just In Time - was the biggest change in manufacturing since Henry Ford. It quickly carried into many business areas.
Instead of having an inventory stockpile of parts to build your product, you ensure your supplier can supply you those parts just when they are needed.
Reduced stock growing dust, reduced warehousing. Yes it's imperative to have supliers who can make their parts quickly upon your order and transport them overnight etc. (this reliance imperative is one of the reasons why the USA transport systems are the world's greatest. Don't try in Australia to get something from Perth to Sydney before next summer...)
We see it in retailing with Amazon... It's just reduced stock on shelves it would have to carry if it was Walmart.
More, if I buy off the Internet and it says 1 weeks delivery it might be because they haven't made the product yet.
In sailing things go wrong but we never know which part is going to break. So we carry spares. But which spares? Many long distance cruisers are advised to carry a spare alternator. But why will that break but not the starter motor? Or the toilet pump? So some people buy all 3 as spares. Plus everything else on the boat: I've heard a number of people with spare autopilots $2,000+ and say it's 'redundancy'... You can just drag another whole boat.
Expendables are different, if course. And so are tools... They're the method to fix or install.
But for the parts themselves I think ( my OPINION) is its a waste of money and critical space to carry ANY spare part (unless I know the one installed is on the way out.).
This does mean that you've gotta get back to Port without that bit of equipment.
Then, in remote areas you may pay a lot for shipping of that part (a friend air-freighted an ENGINE from New Zealand to some tin-pot island in the Pacific.
But in the big picture it's cheaper, saves space etc, to cop the higher cost of emergency replacement.
Anyway, many things are not mission critical so they can be replaced when the timing is better.
There is no doubt that these opinions are strongly held. I've the experience to have them and have seen the downside
It doesn't mean that your alternator isn't going to explode next time you're off the dock... But it could just as well be your dinghy pop a tube.
JIT works in Industry so it's worthwhile to see if it can be applied in sailing.