A few thoughts:
- Bring a large base plate for your car jack. If you get a flat on the road, chances are you will pull over to the side and onto dirt. A thick, wide base plate will help spread the weight of the jack and stop it sinking into the ground. I carry a couple of pieces of 50mm (2 inch) think off cuts; others may use a steel plate with bolt holes to lock the jack to it.

- Even if you don’t know how to use all the tools and spares you carry, it is still worth caring them because the person who stops to help you may know. Manufacturers are all different, I may not have the correct size tool for your car, and items like engine hoses; fan belts etc are very specialized sizes.

-There is also a sort of etiquette to offer a Good Samaritan use of your tools and equipment so that they don’t have to unearth theirs or get them dirty / damaged; having said that people often prefer to use their own tools, but its nice to not assume that they will have the tools / parts or that they will necessarily want to use their own. (On a trip, some of my tools can be buried quite deep in the back and its a job to extract them all. If its raining then this can be extra inconvenient).

-Compressors: There are lots of cheap compressors on the market, but if you have big tyres (eg 4WD, or even some large family sedans) the cheapies may not actually be able to pump them all the way up, and they will be slooooow ! An additional consideration is whether you intent to ever reseat a tyre on a rim. To do this you need a compressor that can deliver a large, high pressure volume in a short amount of time. Basically if you just want to pump up a small tyre, then a quality cheapy is probably OK, but if you have bigger tyres and want to reseat tyres, than you need a bigger, better, more expensive unit ! Activities like 4WDing sometimes necessitate lowering and then raising tyre pressure; with a slow compressor this can literally take hours – not so good if you are with other vehicles and they have to wait !

-Chains: I think its been covered, but chains are generally used for dragging or extending the reach of other straps or cables. For instance we use a chain to drag fallen trees off tracks. It is very abrasion resistant, so even if we have a winch, we put the chain around the tree so we don’t damage the winch cable.
A snatch-um strap is used to recover a vehicle which is stuck. This is a webbing strap with a 20%-30% stretchability and a recovering vehicle can pull it and use the kinetic force to gently pull a bogged vehicle; they are very effective. These can be used as a tow rope and work well, but are quite expensive, so often people use a similar, but less elastic strap. Because the straps are rated for only so many recoveries, and using then can get them very muddy, it is once again etiquette to offer your own and not rely on others to use theirs or muddy it – unless they offer.

Finally consider what you would need if you become stuck in your vehicle for an amount of time ? What if you had to leave your vehicle ? – leave a message on the dash; food; water; shelter; warmth; signalling. Think Car survival kit.