profile icons question

In the profile plugin start.php file I don't understand this over-ride:

elgg_register_plugin_hook_handler('entity:icon:url', 'user', 'profile_set_icon_url');
elgg_unregister_plugin_hook_handler('entity:icon:url', 'user', 'user_avatar_hook');

also we elgg_register_simplecache_view 6 times but it appears only one size gets called.

Can you shed some light on this? If I comment all this out it doesn't seem to effect the plugin.

Thanks in advance.