Why did the pilot inform ATC that he is going to autoland?
From what you said, I'm assuming that the pilot had informed that he/she is going to autoland under VMC.
The main reason is that the autoland procedure requires accurate guidance and the ILS signals may be disturbed by the traffic (both surface and air). So, the pilots want to inform the ATC that they want undisturbed signal. From a Flight Operations Information leaflet from CAA, Qatar:

Most ILS installations are subject to signal interference by either surface vehicles or aircraft. To prevent this interference ILS critical areas are established near each localiser and glideslope antenna.
The ILS sensitive area is protected from all surface vehicles and aircraft when ILS Category II or III operations are conducted or anticipated.

These disturbances may happen when Low Visibility Procedures (LVPs) are not in force and may cause problems for the approaching aircraft:

When the LVPs are not in force, ILS beam bends may occur because of vehicle
or aircraft interference. Sudden and unexpected flight control movements may occur at a very low altitude or during the landing and rollout when the autopilot attempts to follow the beam bends.

Due to these reasons, it is better for the pilots to inform the ATC about their intention to autoland so that appropriate measures can be taken or if it is not possible, the same can be relayed to the approaching aircraft.

The ATC should be informed about the intention to conduct an autoland. Such
information should not be taken as a request for or expectation of the protection of the ILS but is merely given to enhance the possibility for ATC to inform the flight crew of any known or anticipated disturbance.

Also, the ATC has to adjust the aircraft spacing if autolands are being conducted, which may affect the operations.