There are two basic sources for points within the DVC system. Direct from Disney - or on the resale market, buying someone else's points who no longer wants them for one reason or another (sometimes it's not that they don't want them, but can no longer afford them to any number of reasons i.e. job loss, divorce, they're inheirited points and the successor never really wanted them like the bequeathor did... etc, etc).
Disney requires a certain number of points to be purchased when buying direct. The rule is 160 points, although they do offer a couple of resorts where it can be as low as 100.
Buying resale does not have a minimum number to purchase. You find a contract that suits you, you make an offer, and if it is accepted, it goes before Disney for their "Right of First Refusal" process (they retain the right to buy any contract at the same terms that the seller & buyer agrees to). If Disney passes on their right, the contract moves to closing process. If they exercise their right, then they buy the contract and you start searching for a new one.
The lowest number of points that someone can buy resale is 25. But contact amounts go up from there. (That's because that is the lowest number of points Disney will sell directly. Purchasers can get points from 25 up to 49 direct, in any amount, and must pay cash for them. Once a contract is 50 points or more, direct purchasers can finance them if they so desire.). Once you close on a contract, you are a DVC member, and viewed pretty much the same as any other DVC member. Why only pretty much? That comes down to what you can do with your resale points. As of March 20, 2011, those who bought resale could no longer use their resale points for any of the Disney Collection offerings (WDW non-DVC resorts, DLR, international Disney locations, ABD and
DCL). Other than that, you are viewed the same.
When buying direct from Disney, you have the opportunity to select the number of points you want. It does not have to be a set number such as 160. If you want to buy in with 173, they'll sell them to you. Usuallly though most people just buy the minimum, and then if they add more points later, that's when the "off" amounts start to show up. For example not too long ago I saw a resale contract for 37 points. How someone determined they needed 37 more I can't say 100%, though I'm pretty sure it was from whatever their base amount was, plus whatever they needed to secure that "perfect" trip for x number of years down the road.
Knowing how many points you need depends on a variety of factors. SOme people approach DVC as they do traditional timeshares, where they go the same week every year, and like to stay in the same size accomodations every year. Those folks pretty much know they need x number of points.
Then there are those of us who like to mix things up, and enjoy the flexibility that the points system allows for. Different resorts, different times of the year, for different lengths of stay. That can make it a bit harder when you are trying to determine what you "need". Thus the joy of adding on comes into play. And banking and borrowing. But it helps to keep your vacations fresh. Thanks to DVC we've enjoyed every season of the year at a Disney park, and so far 9 of the 11 DVC resorts. We've gone full family, just as a couple, I've gone solo, and we've brought the folks, and in laws and nephews as well. We aren't locked in to one specific week and we love it!
I hope that all helps somewhat. What you "need" is completely up to you and your vacation style -both now, and anticipated style in the future.
ever more enjoyable than park trips) - then yes it is worth while. 
