Recently I was asked why using multiple coupons and printable coupons are important.
Printable coupons allow for savings on a larger variety of items. The inserts can only have so many coupons in them each week, or they would be too thick (hey, it would be ok with me!).
As far as using multiple coupons, or "stacking", this allows for a deeper savings on your item. It can also help you build a stockpile of frequently used items. Many stores allow "stacking" but not all, so it's best to check the individual store's corporate policy before hand, just to be sure.
Some of the tools I use to obtain printable coupons are HotCouponWorld's coupon database, and sites like A Full Cup, My Points, coupons.com or smartsource.com.