If you distribute items good change you use discounts at times to promote sales, move old lines etc. With the trade allowance agreements you can use an off-invoice agreement to control a discount being added to your sales. This is handy because you can define it for a period, track it against a fund, trade some basic analysis of the effect of the promotion with some analysis.
Here is a quick overview.
D365 1611 Update5 (7.0.4475.35138)
AX2012 R3 CU11