Whether I need to rent a car for a day or two because my car's in the shop or a friend is in town, or for a longer period to take a vacation or get around in a city I'm visiting, there are a bewildering array of options to consider: make, model, terms, price.
By researching online I can make an informed decision and get rental companies to compete for my business, rather than keeping me guessing. I can take advantage of special discounts that I'd otherwise not be aware of. And I can get reliable information about the best models to rent based on my needs, and the pros and cons of collision damage waivers and other issues.