Difference between a budget vs offer

Hi,

While i understand the relationship of budget and offer together, i'm a little confused on how they function as its own. For eg; i can create a budget and set the discount rules/criteria where accruals can then be tied to the budget but an offer will need to be tie to a budget. So at what scenario we'll be using budget without offer? Also, if i've a budget with the same criteria set as the offer, will it then 'double' the accrual?

Thanks!