I need this kind of IME on ocassions. For example, i'll need to test some programs which claim they support utf-8.
UTF-8 is only one type of encoding for storing Unicode code points (characters).
When a program claim to support UTF-8 it usually means that it can read or write data in this specific Unicode encoding. To test this kind of a program you will need to write UTF-8 data with another program and give it to the tested program as input. Windows notepad can write UTF-8 files.
With all of the above said, there are certain cases when you will want to input a specific Unicode character in Windows using the keyboard. For this task an Unicode code points Input Method Editor will be very helpful.