As mentioned, when you buy a contract, whether resale or direct, with the same UY and deeded the same way, it stays in your membership as an add on contract It does not have to be the same resort.
If you get a different UY, or have different owners, then it is a brand new membership and not considered an add on contract. Again, doesn't matter whether you buy it resale or direct. One of my memberships currently has RIV direct points and SSR resale points in it.
Usually, they do not give current members any incentives when onsite. Now, you may be able to get them to award you some FPs, but they are not usually offered. Prices direct are what they are and sold out resorts are the same price no matter how many points you decide to buy.
If you want to get RIV or AUL...and I believe right now CCV...there are incentives that kick in the more points you buy. I do not believe any of them offer discounts for less than 100 points.
Obviously, the benefit of resale is that is is going to save you money if you are looking for a larger number of points. The drawback is those points can not be used for RIV and future resorts if that matters to you.
One benefit of direct, even though it will cost more, is that you can get a smaller number of points easier. Small contracts are not as easy to find on the resale market. It is also easier for Disney to get you the UY that you want vs. resale where you have to wait for one to show up for sale.
Good luck!