The idea that die cast model cars are valuable collectors items is probably true, since they have been classed as collectibles since the 1930s. In 1934, Dinky Toys brought out die cast metal toy cars, which presumably parents brought for little boys, who grew up to be men who still liked the cars, and turned play into collecting.
Anyway, contrast these collectibles with Beanie Babies, which turned out to be a fad. Nowadays you see them mainly in thrift shops, not in display cases or mounted on lamp stands. You do see die cast cars in thrift shops, but they are segregated and individually priced, not thrown in with other small toys as the cute little baby animals are.
Why collect car models? Well, why collect anything? You either get it or you don’t. But collecting these little (3″) or not so little cars (10″ – 12″) does make sense to a lot of people. To begin with, they may have started as toys, but they soon developed in to scale models, or miniaturized representations of actual vehicles. They are true to life in proportion and detail, thereby rising above the toy level to the distinction of true models.
Die cast has traditionally meant being made of metal of one sort of another, or a combination of metals, forced into a mold while molten, with much pressure to fill the mold and to allow the metal to harden. Today you can also have die cast (or diecast, since the terms are used interchangeably) plastic cars. The metal ones are the collectible ones of choice, it seems, although early plastics have definitely become collectible, so who knows?
The models are still being enthusiastically made, and the age of the model does not seem to automatically raise the price. As in the original, actual car, the appeal is in the eye of the beholder. A model car may be a remembrance, of the VW bug that was your first car, of the memories that are aroused by the sight of a 1969 Mustang convertible, or of sitting on the floor watching the General Lee fulfill all your six-year old ideas of what a car could do.
It may also be as close as you will ever get to owning a Ferrari, say, or a Lamborghini. If you really always wanted one, somebody is sure to give you a model for Christmas sooner or later. If you really rate, it may be a premium model with doors that open, a steering wheel that turns not only itself but the wheels as well, and a working suspension. If your friends really get carried away, they may get you one with a working internal combustion engine, although they are venturing close to the toy line here.
As in all collecting, it is wise to focus. You may want to start with police cars, with all the versions of the Batmobile, with cars from famous movies, or from the era of history that interests you most. The choice of models in all these categories and more is very large, and there are many manufacturers who used to and/or are still making these small vehicles.
It can turn into a hobby, too; you may want to put your cars into display cases and hang them on the wall, but you may want to restore them, airbrushing on bright, glossy colors, or set them in dioramas with little, die cast gangsters or carhops or service men just the right scale to match.