There are a few reasons to grind your own spices:
1. It's cheaper to buy whole spices than to buy it ground.
2. Whole spices last longer than ground spices. As soon as a spice is ground, it starts to go stale. So not only do fresh ground spices taste better, you can buy whole spices in bulk, which, again, makes it cheaper.
3. Grinding your own spices lets you create your own spice mixes. It gives you more control over the quantity of each spice, and it allows you to make decisions about the type of spice - for instance, if you want toasted cumin, you can toast the cumin before grinding it, instead of having to buy a whole jar of toasted cumin, and a whole jar of non-toasted cumin.
4. Similar to #3, sometimes you need whole spices, sometimes you need ground spices, and it's easier to just have to buy one kind.
5. Some ground spices have anti-caking agents in them, like silicon dioxide. Grinding your own spices means that the spices you're using are just spice.
6. Perhaps most importantly, it's more fun.
When Matt and I made the Black Bean Pineapple Soup-Stew-Chili, the recipe called for Ancho Chile Powder. The stores we shop at don't carry a lot of chile powder - they carry what Matt calls "Chili-with-an-i" powder - a mysterious combination of chile powder and other spices. "Chili-with-an-i" powder is fine when you're making soup or chili and just want to add some flavor, but it doesn't let you pick out what kind of chile pepper you want to use. And anyone who has tasted the difference between a Hatch Chile, an Ancho Chile, and a Chipotle Chile knows there is a big difference depending on what you want to use.
So what to do? Luckily we were able to find some dried Ancho Chiles, and making chile powder is a pretty easy process.
The first thing you want to do is take the stem and the seeds out. In chiles, the flesh has most of the flavor, and the seeds have most of the spice. If you want to make a hotter chile powder, leave some of the seeds in. Knowing how many to leave in is somewhat trial and error. I would be lying if I said Matt and I never had to get rid of a dish because our chile powder was too spicy. So if you don't have the luxury of ruining dinner, start with few or no seeds. My theory is that you can always add tabasco sauce if it's not spicy enough.
Once you've removed all the seeds, tear the chile up into small pieces - about the size of a dime. Put the pieces in a spice grinder. We use an old coffee grinder. (Do NOT use the same grinder for your spices that you use for your coffee, unless you like ancho-flavored coffee, or coffee-flavored cumin). Add a pinch of salt to help bring out the flavor and facilitate the grinding.
Last step - grind away! Keep going until you have it at the consistency you want it.
This whole process took less than 5 minutes - I was able to get it done while the peppers and onions were cooking for my soup.