Many slot car hobbyists use diecast cars as part of the scenery on their layouts. There are lots of relatively inexpensive diecasts that provide an easy way to populate the parking areas and paddock with road cars, trucks, vans, RVs and other vehicles typically seen at race tracks along with the race cars. These low-end diecasts vary in scale, even among those that are supposed to be 1/32 scale. This is because they are not scale models for the adult collector but toys. Most, if not all of them have pull-back "motors" in them to provide play value for children. Because they are toys, mostlly meant to sell from countertop assortments in many kinds of stores they are often made not to a consistent scale but to a consistent size that fits the standard-sized assortment boxes they come in. This typically means that smaller cars may be right on or close to 1/32 scale while larger cars, trucks, and other vehicles will be made to a smaller scale, the bigger the vehicle the smaller the scale.
What we have tried to do here at VLH is to find diecasts that are fairly close to the two scales of slot cars we sell, 1/32 and 1/43. In our product listings we'll tell you which of the two scales they are closest to. For example, a VW Beetle or a Cobra is very close to the size of its slot car counterpart in 1/32 scale while a Chevy Silverado pickup truck is much closer to 1/43 scale. Many other cars are less close, like 1/34 or 1/36 scale, and can be used where they are not going to be right next to one of your slot cars, such as in a spectator parking area back a ways from the track. In fact, you can use vehicles (as well as buildings, figures, and other objects) in various scales to create the illusion of distance through what model railroaders call "forced perspective".
And, of course, if you have kids too young yet for slot cars and you just want to buy them some cool toy cars to play with, these diecasts work just fine for that.
It's worth keeping in mind, by the way, that not all slot cars are true to scale. Slot cars, however, tend to be oversized. This is because some manufacturers try to fudge the dimensions of cars that are different sizes in real life but meant to be raced together so they have similar performance as slot cars or so the bodies can fit over a common set of mechanical components. This is almost always done by bringing the smaller ones up to the size of the larger ones. The Fly/GB Track classic cars are a well-known example.