Q. I am a novice at woodworking. I have a table saw, a table router, and a cordless drill. What exactly does a jointer add to a shop? I have not even settled on what projects I wish to do.
A. Michael Dresdner: "A jointer, which does the same task as a hand plane, will cut one flat side on a board. From that, you can use the flat side as a reference to cut an adjacent flat side at right angles (also on the jointer, or on the table saw) or cut a parallel flat side (with a planer.)"
A. Rob Johnstone: "Jointers are versatile and important tools for stock preparation and shaping. A jointer, properly set up and used, will allow you to create a dead straight edge on a piece of lumber ... which makes for a good glue joint or a straightedge to rip from on your table saw. An adjustable fence on the jointer will allow you to chamfer an edge or create a bevel on a piece of stock. (Some jointers have a notch that lets you form a rabbet on the edge of stock, though I must confess to never having used a jointer for that purpose.) You can also use it to take a bow out of stock before you run it through a planer. (This is a more advanced technique.) I've also made a whole bunch of uniform, tapered table legs on a jointer using a tapered jig clamped to my outfeed table. While the variety of lumber available today that is surfaced on four sides makes jointers less important to some degree, I would very much miss it in my shop."
A. Lee Grindinger: "A jointer's role in the shop is to flatten the faces of solid stock and straighten edges. As you delve more deeply into this craft, you will realize the importance of flat, square and true stock as you construct a piece of furniture. A slight twist in a board will make it impossible to true that board and lumber, as it is sold, is not generally flat. The first part of sizing a board is to flatten one face in preparation for the planer (thicknesser in some parts of the world). You can flatten boards using hand planes, and many still do. In fact, the jointer is the machine that was devised to do the tasks associated with hand planes. Don't rush out to buy a jointer, though; you'll know when you need one as your tolerances shrink."
Q. "I have a new jointer that I am in the process of tuning. The problem I have is that the infeed and outfeed tables are not aligned in that they are high at the ends. The manual is totally useless. Experimenting with the gib screws shows that loosening will lower the ends of the tables. The problem is that when I tighten the table lock screw, it raises the end again. I don't feel comfortable operating the jointer with the tables loose. Any ideas would be appreciated." 
A. Simon Watts "I see no need to adjust both tables - all you care about is the alignment. So leave the infeed table alone; then when you lock it, it won't move. Adjust the outfeed table, using an engineers straight edge, and pack the ways, if necessary, with brass shim stock. If you don't have any, aluminum foil works almost as well. Be sure the outfeed table is exactly at the height of the cutting circle of the knives."
A. Rob Johnstone "The tables of the jointer should be exactly aligned to each other in their horizontal relationship. The gib is the joint between the tables and the jointer body and controls the relationship of the tables to one another. It is not uncommon to put a shim in the gib to adjust this relationship. Put the shim between the jointer body and the outfeed table and once you have that baby correct ... tighten it up and leave it alone. This is a tricky bit of work and you don't want to have to do it any more than is absolutely required."
Q: Why is the outfeed table on a jointer adjustable?
John Brock: So it can be set to the height of the cutters and eliminate snipe.
Richard Jones: To make fine adjustments so as to get the surface of that table tangent to the cutting circle described by the cutters in the cutter block.
Carol Reed: As you sharpen cutters you will find that you may need to adjust the outfeed table to align with them.
Q: My jointer does not have an adjustable outfeed table. What do I have to do to eliminate snipe?
Tim Inman: Set the knives in the cutterhead to the exact height of the outfeed table.
Richard Jones: Set the knives so that the circle described by them is just a hair above tangent to the outfeed table.