The only reason to fill out that field is if you want to use your Minecraft avatar on the Platform. In which case you also have to click on the check box to enable it. Otherwise it will use whatever avatar you have associated with your Gravatar account. If you don't use your Minecraft avatar or don't have a Gravatar account, then you'll get a random avatar.
You can also associate your Minecraft username on the forums, but all it does is show it on your profile and on your posts(as Jaari linked to above). It does not change your avatar.