That's not entirely true. The alpha channels are merely one of the many ways to save your masks.
But to do what you wanna achieve in Photoshop:
For solid / objects with large areas of colours e.g. body, shirt, etc
1) Create the initial mask
a) use various combinations of the marquee, ellipse, etc selection tools, holding down the shift to add to selection.
b) draw vector paths using the pen tool, then convert it over to a selection. It's not easy to draw, but it's the most powerful.
2) QuickMask mode & edit the mask further, using airbrush/painting techniques to achieve smooth graduations at the edges, pepperpot holes, honeycomb patterns etc
For hairs, grass, or terror of terrors, wheat swaying in the wind in front of a green leafy background:
1) Use the Extract tool
Something OT but why is the alpha channel so useful?
Suppose you have a HUGE 100 MB high-res image. Unless you have a very powerful workstation with tonnes of memory, creating a mask on such a big image can tax your system and waste your time. So resize the image down to something more manageable, ensuring you maintain proportions, and then create your mask on this copy. Save the alpha channel, and load it in your original high-res image. The selection will scale accordingly, and you can then print out your image at the highest quality possible.