You use basically for emphasis when you are stating an opinion, or when you are making an important statement about something.
This gun is designed for one purpose – it's basically to kill people.
设计这把枪只有一个目的–主要用于杀人。
You use basically to show that you are describing a situation in a simple, general way, and that you are not concerned with less important details.
Basically you've got two choices.
大致说来,你有两个选择。