They work in conjunction with one another and are used primarily for rough sawn lumber. However, unless you can get straight, flat and square store bought stock, you will need a jointer first! WHY? It's a safety issue, because the table saw requires straight, flat and square edged stock on the table and against the fence.
WHY? When you rip a board down it's length the edge against the fence can't be curved OR you will cut a similar curve on the edge. A jointer OR a straight line rip jig will fix that issue.
What about straight and flat? If the board should twist during the rip, it will bind the blade stalling the saw OR worse yet, kickback at the operator. A jointer is the ONLY machine that can make a flat surface ... easily. Yes, a jig can be constructed for using a planer to make the top edge flat, BUT it's a PITA to use and setup. I know I've done it.
So, if you can buy straight, flat and sqaure edged stock, go for it. If you jave to pay a shop or mill to get it, then you have to weigh the cost benefits of getting a jointer first, then the "thickness" planer. The thickness planer, it's proper name, is the only machine that can make a uniform thickness board ... easily. Thicker stock can be planed down easily into thinner stock OR resawn boards off the mill or bandsaw can be planed down to "project appropriate" thicknesses.
Introducing a good resaw bandsaw will open up all sorts of new and great possibilities when combined with a jointer and thickness planer. Ask me how I know......
I haven't purchased store bought hardwood in years for my projects since I had Oak and Maple trees milled into rough sawn lumber right on my land years ago. I realize that many woodworkers don't have that advantage and must go to a mill or decent lumber yard for their hardwoods, even Pine and Spruce. I will get Poplar from the box stores when it suits the project.