They might "obviously need" this data if they want to synchronise the improvements along all your computers and devices.
Or to make the general dictionary pick up on new terms automatically and give them to other windows users.
Edit: The full quote is actually:
> We also collect your typed and handwritten words to improve character recognition and provide you with a personalized user dictionary and text completion suggestions. Some of this data is stored on your device and some is sent to Microsoft to help improve these services. You can turn the Send Microsoft info about how I write setting on or off in Settings.
Which means they don't send EVERYTHING so it's not really a keylogger (unless you also count the keyboard-helping stuff keyloggers?).
Furthermore on that link, it seems to me that the data they would be sending to their servers is the ones about the other services like speech/handwriting to feed into their system to machine-learn recognition.
And it is all turn-off'able which is a plus.
It seems like most people will assume the worst, I am not saying you're wrong but it doesn't seem like some "malware" thing like other people are saying..