A main pro of buying a mattress and bed frame together is getting a workable set at once, since both pieces should function well as the main parts of a bed. A major con of buying the two pieces together is that more money must be spent at one time. This may mean going without the needed bed until money for both pieces can be earned. Lack of convenience can also be a con of purchasing both a mattress and bed frame together since many stores don't sell these as a set. Yet, a pro could be that time may be saved in the long run by buying both pieces from the same store and having them delivered or taking them home together.
Of course, if a person already owns either the mattress or frame, then buying them together wouldn't be advantageous. It would then be more convenient to be able to buy only the needed piece to complete a bed. Some people on a tight budget may also prefer to not buy a mattress and bed frame together so they could purchase the first item new and the second used to save money. Mattresses and other "soft" furnishings that are previously used could harbor bed bugs, whereas hard, solid surfaces such as metal bed frames are less likely to hide the creatures. A con in this case though is that some wood frames may contain bed bugs, and it may be more difficult to find a used metal, rather than wooden, bed frame.
Having a bed all ready to use is a strong pro for many people who choose to purchase both the frame and the mattress at the same time. In this way, the pieces can be set up in the desired bedroom and be used immediately. If just one piece is bought when two are needed, the other one may need to be stored somewhere or will just take up space in the home while being unusable. Another con of not buying a mattress and bed frame together is having to spend time sourcing out the other piece and making sure the measurements are compatible. If a person keeps putting off the task or has trouble finding the second piece, then the money spent on the unusable frame or mattress becomes a waste of funds.