Big question.

What you are really getting at is disribution channels. How many layers are there and who controls what flows through them.

As a brand, if you can remove a layer of distribution by being your own distributor, you save money and can be more cost competitive. If you also remove the IDB (to varying degrees) it lowers cost again. Owning your own distibution channels allows you to cut cost all the way back to the manufacturer.

For the IBD it can cut both ways. If you are in a smaller/weaker market the brand can support you with product on decent terms, POP materials, and other help. Great.

If you are an IBD that is seeing real growth and wanted to expand into other products it can be very limiting. There are a lot of conditions put on the IBD and they can lose a brand/product line for not adhering to them.

For suppliers it doesn't matter in some respects beyond the fact of how much a PITA it is to deal with big companies. In some ways it can simplify things as it consolodates orders and thereby production which also simplifies things like buying (raw materials), production scheduling, hiring and logistical considerations. The factories don't care who's buying as long as they're buying.

For end users it also cuts both ways; It brings greater access to a wider variety or products than would otherwise be available in smaller markets but it can limit the range of products readily available in a stronger market.

The process streamlines channels and, in many instances, allows brands to increase margins and also offer products that are more price competitive. Not a hard and fast rule but more of a general one.