Most miracles are natural phenomena with awesomely good timing. The Ten Plagues are a notable exception. Here the laws of nature are turned upside down to help free the Jews. Each plague is an open miracle, because each one represents a fantastic manipulation of nature. The laws of nature are turned upside down to help the Jews.
Open miracles are a very important part of early Jewish history.
After the destruction of the First Temple they're going to cease, although arguably the Jews couldn't have survived this long without continual hidden miracles. If He wanted, God, an all-powerful being that He is, could have made all the Egyptians drop dead on the first encounter with Moses, or He could have frozen them in place, then all the Jews could have packed up and left in five minutes. To explain why the Ten Plagues had to be, we need to first explain the Jewish view of miracles in general. Judaism holds that nature does not act independently of God, but, at the same time, God created the laws of nature and does not interfere with them. God is certainly capable of doing whatever He likes, but He doesn't play around with the physical world and its workings.