Why 1 Freshness.
Beans are best between eighteen hours and about a week and a half old.
Roasting yourself ensures that you can hit this peak - and you know when they were roasted. Different coffees break this general ideal. Some will reach their peak as much as three to four days after roasting but you have no way to judge this unless you are roasting your own. You can roast as little as 4 ounces so you are not stuck with half a pound of the same bean to drink in a fortnight. This is particularly useful when you want to try home blending or cupping and want small quantities of several beans.