Presumably you've made positive connections with your fellow teammates and feel some sympathy for how they may suffer a bit with your departure. You're giving notice so they (and you) can mitigate that suffering. During that time you may not be doing much work, but answering questions, typing up arcane knowledge you have about particular areas, etc.
Not burning bridges. If you hope to every use anyone at your current company as a reference in the future, give decent notice. The last impression you leave is likely what will stick.
In the end, I'd think of it as following the Golden Rule. Even if they don't, if you'd like to work at a company that would provide severance in the event they needed to let you go (basically giving you "notice"), then do the same. The world is what we make it.