Sentences with "will be" like these:

It will be fixed

...mean that someone will do something. You avoid saying exactly who will do it by using this structure. 

You might speak this way if you don't know who did an action:

It's been fixed.

Or if you want to hide who did it:

I was told not to say anything.

Another reason to use "be done" is for politeness. Instead of directly telling someone what to do:

You have to fasten your seatbelt.

...you can instruct people more politely like this:

Seatbelts must be securely fastened.