New cars have a lot of parasitic/vampire battery drain going on, and if you don't drive them regularly and for more than just short trips to keep the battery well-charged, it can be a problem. On top of that, OEM's are putting in smaller and smaller batteries.
The parasitic draw notion kinda depends on the era/make of the car. I work on Audi’s and BMW’s, and the older cars (especially higher optioned cars) did have a slightly higher acceptable draw. But, as technology advanced, the cars have (theoretically :laughing: ) gotten more intelligent and are able to both allow non-essential stuff to go to sleep as well as physically shut off power supply to certain branches of the systems. So you can have a car with all the bells and whistles drawing 20mAh when it’s sleeping.
Flip side of that is how true the other part of your statement is - Frequent short trips are hard on the batteries as there are so many consumers in action that the battery may not get replenished from all the energy used to start it.
For my own car- In the rear side compartment, I keep a small ratchet strap for cargo management, tow strap for others just in case. Under the floor I keep a set of jumper cables (as others have mentioned - if I help someone, I’ll be doing the attaching, thank you...), and towels to put down if I carry anything dirty.
During winter months, I also keep a full set of winter wear - snow pants, extra coat, hat/gloves. Used to never worry about it as I was indestructible. Then one day on the way to work I got rear-ended, and my exhaust got folded up so I couldn’t (well, didn’t want to) run the engine to stay warm while I waited for the tow truck. Had I not been able to sit in someone else’s car most of the time I waited, I’d have been in trouble.
Also for winter there’s the default ice scraper/brush (I prefer the super cheap wood handled ones - they just work best for me), and a 6” plastic taping blade (like for painting houses) - that thing is awesome for clearing large amounts of ice. Strong enough to get the job done, but flexible to conform to the glass. Clears a lot more ground than a standard scraper.
Summer months I keep some microfibers and waterless wash for cleaning bird bombs off.