Yeah, crafting is kind of a pain but look at it this way:
Take anthonydido's example: You have a DC 18 check to make a belt of strength without using the spell Bull's Strength.
Assume you are level, what? 5? and assume you took an 18 int after racial.
Spellcraft = 4 for int + 5 for level + 3 for trained = 12. So you take 10 while crafting and that gives you a 22. If you have a 20 int or are level 6 (or both) you could speed up the daily crafting by adding another 5 penalty making the DC 23 instead of 18 (your take 10 score would be a 23), but only taking 4 hours to craft thus freeing up more time in your day.