I don't think miles and cashback are the primary benefit of a credit card, although they're handy. A credit rating on the other hand is very important: at least in the US, finding housing (including apartment housing) is seriously complicated by having bad or no credit, and the same goes for buying vehicles or anything else customarily paid for on an installment plan. Making major purchases on credit is a good deal if you think hanging onto the money is worth more to you yearly than the APR, which isn't unlikely if you're investing; basically it's leverage.

If you find credit cards morally objectionable or are absent-minded enough not to always pay them off on time, can probably drop the card once you've been approved for an automotive loan or something comparably serious. I use mine to handle gas and certain other predictable expenses.