No, you don't need them and there are some really good reasons NOT to send them.
1. They cost money to produce, whether you're going for a simple card or a magnet. Add the postage, the envelopes to their cost, and you'll be surprised how much save the dates really cost.
2. They add another thing to your "to-do" list, which is long enough already, isn't it?
3. Your plans may change; your budget may shrink, your dream venue may not end up being as big as the one that you sorta like. No one wants to be in the position of un-inviting people who thought they were coming.
4. No one can keep a secret; and you can bet that anyone who gets a save the date will tell all their friends, some of whom were hoping to get one too. By not sending them, you have a chance to really tailor your guest list to your final budget.
Budgets, styles and relationships change. You made the most important commitment already; don't commit to your guests way ahead of time. The ones who want to come to your wedding will; the ones who can't? Won't. You'll be just as married (and slightly more flush) at the end of the (wedding) day!