Once you understand how cloth is made, and how much time a weaver invests in the process, you'll appreciate that weavers might appreciate any time-saving measure. A loom that creates a shed will greatly speed up weaving.
Looms for Needle-weaving
Remember that the weft yarn needs to go over and under the warp yarn in each row (pick) of weaving. You can manipulate the weft yarn over and under with a needle or your fingers, say on a simple cardboard loom or a potholder loom. This technique is called needle-weaving, and it's often the first method weavers learn. When you create a 4" x 4" square on our Zoom Loom, you'll do it with needle-weaving. Here the needle, threaded with blue weft yarn, goes over and under yellow warp threads. You can weave color and texture patterns with needle-weaving.
Looms That Use Weaving Tools
On other Schacht looms, weavers make a shed, then weave across with the weft yarn on a shuttle or needle, and beat this pick (row of weft) into place.
To make a shed, the weaver raises half the warp ends at the same time; the other warp ends stay down. The weft yarn now has a clear path to travel across the loom—as it passes through the shed, it automatically goes over the "down" warp ends and under the "up" warp ends. On the next pick, the weaver raises the other half of the warp ends. The interlacement of warp and weft happens quickly, with little effort.
In the diagrams below, you see pick-up sticks making the shed on a School Loom. A needle carries the blue weft yarn across, or you can wrap yarn on a shuttle to weave across.
A pick-up stick threaded under half the warp threads.
The pick-up stick on its edge, making a shed for the weft.
A second pick-up stick threaded under the other half of the warp ends.
The second pick-up stick turned on its edge, making a shed for the second row of weft.
Our looms designed especially for warp-faced or weft-faced fabrics make the shed with weaving tools or heddles. (The assembly manual for each product explains in detail how to set up the loom and weave on it.)
- Holes make the shed in Card Weaving Cards. Thread warp yarns through these holes in a pattern. As you turn the cards, a different shed opens.
- The Inkle Loom for warp-faced bands use string heddles or Texsolv heddles. You push down or lift up these heddles to open a shed.
- The Lilli Loom and School Loom come with weaving tools. You "weave" a pick-up stick over, under, over, under. Then you turn the stick on its edge to open the shed.
- The original Tapestry Loom comes with wood heddle rods; you attach string heddles or Texsolv heddles to warp ends, then loop the heddles on different rods. Then you can raise a rod or rods to open a shed.
- The Arras Tapestry Loom has Texsolv heddles that attach to a shedding device: simply turn the handle to raise or lower warp threads. If you put the Arras Loom on the Arras Stand with a Treadle Kit, you can operate the shedding device with treadles.
These videos show the differences between the Inkle Loom, School Loom, and Arras Tapestry Loom:
Looms That Make Sheds
Looms that make balanced fabrics have built-in ways to change sheds (though weavers can also use tools for pick-up patterns and finger-controlled weaves). Here the looms do more work so weavers can make a huge variety of weave structures, in long lengths of fabric. Imagine using tools for every pick on a 6-yard warp! Weaving would get much slower, and it would be easy to make a mistake.
For these types of looms, the weaver can warp for a complex pattern. Then the loom takes responsibility for making and changing sheds. Weaving proceeds very quickly, even in a patterned textile like this one:
On a rigid heddle loom, the reed changes sheds very quickly for plain weave. In the towels above, pick-up sticks also helped.
It's easiest to see the shed when you look at a loom from the side. Here you can see a Cricket Loom warped with pink and yellow yarn. When the rigid heddle sits in the neutral position, there is no shed.
Set the heddle in the up position, and half the warp ends will be raised. 
Move the heddle to the down position, and the other half of the warp ends will be raised.
Weaving on a Cricket looks very similar to weaving on a Flip:
Treadles or levers raise shafts on the most sophisticated type of loom, the shaft loom. Warp ends go through heddles on each shaft. Raise the shaft, and you raise the heddles.
Shaft looms can weave the most elaborate patterns and weave structures without any tools. It will take you more time to warp a shaft loom than a rigid heddle loom, but once all the warp ends are in place, you can weave very quickly. This towel, featuring blocks of twill, was woven on an 8-shaft loom.
It would be possible to work this pattern with pick-up sticks, but it would be a slow process!
In this video, the woven fabric has color and texture patterning—levers on the shaft loom change the shed: