They make money on the financing, so are likely to be more willing to negotiate on price if they think they will make it back on the financing.
Doesn't mean you can't can't _pay_ cash, but 95% of dealers you don't want to be too upfront about that.
But don't let them know that until they're getting ready to fill out a loan application.
Individual sellers will more likely give favor to cash in hand, at least I know I would rather have cash in hand for my car.
Paying cash is generally[] not better than financing through the dealership, but is much better than bringing your own financing.
[] There are a couple more factors that I haven't seen mentioned, specifically that paying cash is generally a lot faster. For the dealership, there isn't necessarily a benefit, but for the salesperson, a quick sale means they might get to slot an 'extra' sale in for that day, so it's worth it to them to play around with their commission on what they might consider a 'gimme'. If paying cash, it is best to do so on busy days so that there are other 'fish' for the salesperson to go to next. If you're the only person there, there's no real sense in urgency.