fillers are the little items you use to get your $$ total up to a certain point to use a coupon, and to get your item count high enough to use all of your coupons.
Heres a few very basic scenarios
if you are buying items to earn EB but your total is going to be $19.02. if you buy a filler that costs $1 you can now use a $5/20 coupon. so spending that extra dollar saves you 4.

also, if you are using more coupons that items, the register is programmed not to accept them. The "register logic" doesnt understand that there are allowable things like cvs and mfr coupons used for same item etc. it just thinks 5 items, 5 coupons and the cashiers can only force so many "extra's through before the register refuses to take any more.
so by buying a few items that cost pennies, will allow you to use more coupons. but check the register logic post to get the full detail. its very clear and detailed.