If you no longer need an email account on your iPhone, iPod touch, or iPad, say if you changed schools, jobs, or service providers, you can easily delete. If you're simply going out of town, on vacation, or otherwise off-the-grid and you don't want the distraction or data demands that might go with mail, you can also temporarily turn it off and give yourself a break.
If you've got multiple email accounts set up on your iPhone or iPad, the Mail app will automatically reply from whatever account an email was sent to. But what about when you're composing a new email? If you've got one account you use more than others, you can set that as your default account for composing emails. If you ever want to send from another account, you can still do that too.