A regular bed supports a structured mattress on a rectangular frame of rails, relying on the internal rigidity of the mattress to keep it from sagging. If you have a structureless mattress, you will need a platform bed if you want any chance at getting a good night's sleep. Adding a plywood sheet to the rail frame may be one solution to this problem.
The main difference between these two bed types is how much support they provide beneath the mattress. Traditional beds offer no support, with the exception of a central rail for larger queen and king mattresses that will not sag. A platform bed offers a solid platform beneath the mattress for extra support. However, that platform often rests on a rail structure very similar to a traditional bed. This makes adding a plywood sheet for support easy to do.
Adding plywood to convert your traditional bed to a platform bed is a simple process. You measure the internal dimensions of your rail frame, then cut a plywood sheet to match. For larger beds — which may be larger than easily available plywood sheets — you may need to use two sheets. Before placing the plywood in position, it's a good idea to thoroughly sand the raw wood. Not doing so risks snags and splinters in the bottom of your mattress.
The chief advantage of a platform bed is that it will support a mattress without internal structure. If you have a top mattress with no matching box spring or an unstructured futon mattress, it will slide right between the rails of a traditional bed. The platform will support that kind of mattress and help reduce sagging in an older structured mattress that has seen better days.
Air flow and comfort are two distinct disadvantages of making this conversion. Air flow is a factor because traditional beds allow air to pass beneath the mattress. Without that air, mold and mildew are a bigger risk with platform beds. Platform beds are also noticeably harder than traditional beds. The mattress cannot sink and conform to your body as well with a solid platform beneath.