Note: Quotes below are in reverse order.
A planer will give you two parallel faces. A jointer gives you one flat and straight face. With that face against the fence you can make an edge square to the face and straight. Then it goes through the planer so that both faces are parallel and eventually to the appropriate thickness.
If you need to face joint couldn't you put it through the planer and just barely take any material off?
Yes. You can face joint on your planer to get a flat face, as long as the underside of the board is flat.
If the underside of the board is not flat, then the board may move as passes through the planer. There are solutions to that issue. In some cases, you can put the board and a sled and support it so that it does not move as it passes through the planer.
You got a perfectly flat face with your planer, but now the question is, "How do you joint the edge?"
You may have forgotten that a jointer has something extra that a planer does not have: A perfect 90 degree fence. You use that fence on the jointer to help you make that perfectly straight, perfectly square 90 degree edge on the side of the board.
How do you get a perfectly straight, flat, 90 degree edge on your flat board if you have a planer, but no jointer? Most people use a table saw as a substitute, but that has its own issues.
In theory, you install a blade that makes very clean rip cuts, set it to a perfect 90 degrees with respect to the table, lay your new flat side of your board on the table, set the rip fence, and trim off the ragged edge, leaving a perfectly straight, smooth, square 90 degree edge.
The problems appear if the OTHER edge of the board is not straight. If you put that non-straight edge against the fence, then the board may shift as the non-straight edge goes past the end of the fence. That can be very dangerous, and could easily lead to a kickback. The solution is to make and use a "jointer sled", and you still have the question of how clean an edge the table saw can leave on the board. Some people choose special "glue line rip blades" to get a cleaner cut.
There are other solutions to squaring an edge:
* Use a router table. Some people make an economic jointer by setting the infeed and outfeed fences with a slight difference, like a jointer. The rotating router bit acts like jointer blades. It does not have the ease of use or precision of a true jointer, but it works for short pieces. Router table fences and tables are not large, so jointing a long board has its own issues.
* Use a hand plane. Clamp the board and plane it by hand. This requires skill, and acquiring that skill requires a lot of practice. I have tried it many times, but I am not close. Simply holding a hand plane square along a long board is not easy. Planing straight is not easy, either.
* Use a hand plane. Construct a shooting board or build a jig to keep the board at right angles to the hand plane. The hand plane has a 90 degree side to help align it for the shooting board or jig.