Such racks can be the general purpose type such as those that are standard equipment on many SUVs and some crossovers, or the more specialized removable systems with purpose-designed components to hold and lock-down sports equipment such as skies, bicycles or kayaks. The removable systems such as those offered by manufacturers like Thule and Yakima can be purchased for practically any model or style of vehicle, but they can get pricy if you add all the specialized equipment carriers available for them. Factory-standard or optional racks can also work, but some of these are not rated to carry the loads you may want to carry, while the best of the removable systems are much stronger.
When fitted on smaller vehicles, roof racks free up passenger space inside by allowing you to securely strap your gear and supplies overhead, where it's out of the way. As a means of carrying back-up vehicles, like bikes, canoes or kayaks, or shelter building materials like poles or lumber, roof racks are invaluable, because even with a large pickup some of these items are awkward to carry securely. A good rack system can often eliminate the need to pull a trailer, which adds its own set of complications when bugging out of a SHTF scenario.
So what kind of rack is best for your vehicle? One of the most versatile systems I've ever used is this basic set of Thule cross bars that I've owned since 1988. I have been able to make these work on several vehicles I've owned over the years, from sedans to sports cars and pickup. These make carrying canoes or 17-foot sea kayaks such as this one easy - even with the smallest compact cars:
I've never bothered with the specialized cradles for kayaks and attachments for other gear, preferring to simply tie down my load directly to the bars, using padding if necessary to protect delicate items - which my kayaks are not - as I build them to use, not look at.
These simple crossbar racks are rated to carry 165lbs. That's more than most people will need to strap on top of a vehicle, but I've certainly pushed them over the limit hauling lumber, causing them to flex but with no failures so far. They are available in lengths from 50 to 96 inches, making them adaptable to a wide range of vehicles. The mounting systems are sold according to your vehicle, and range from old-style vehicles with rain gutters to the sleekest, aerodynamic roof profiles of today. The bars can also be fitted with adapters to make them work with the fore and aft roof rails that many vehicles come with, but without crossbars except as an expensive manufacturer's option. More information about the fitment of these racks can be found on the Thule website. Amazon stocks the load bars as well and most of the fitment options you might need.