You need an external hard drive to support backup, and may also wish to use one to transport big collections of data, to archive data for long periods, or just to add storage space. Prices of disk drives have been dropping amazingly for years. I think I remember speding $200 for a 1GB drive about 12 years ago. Now a 1TB drive, 1000 times as big, costs about $100.
The question is, what drive to buy? There are many manufacturers, and you can find positive and negative information about lots of brands. One person will have bad luck with a certain brand and never use it again, and someone else will never see a problem.
I have bought several drives from LaCie and had no problems with them. They come ready to use, work well with the Mac, and are neither the cheapest nor the most expensive. They have nice metal cases and plug into things with lots of different interfaces.