Is the ILS only used for landing when weather conditions are poor?
The relevant question is not whether or not the ILS system is used at all, but when the pilot switches from ILS to visual.

At the beginning of an ILS approach, the ILS signals function as a navigation aid, helping to make sure that the aircraft is lined up correctly with the runway from a longer distance out than the runway itself would be visible clearly enough to the naked eye even in good weather.

At some point during the approach, the runway will be clearly enough in sight that the pilot can actually land the plane visually. The latest point at which this is allowed to happen is the "decision height" -- if the runway is not reliably in sight by then, a go-around must be executed.

The lower the decision height, the larger demands are placed on the accuracy and reliability of the ILS equpiment -- losing the ILS signal due to interference for a few seconds while still six miles out is much less critical than losing the signal for a few seconds right before touchdown.

It is possible (if all the involved equipment is maintained and certified to high enough standards) to have a decision height of 0 feet so the ILS/autopilot flies the aircraft all the way to touchdown, but doing this requires all other aircraft moving around on the airport surface to observe extended safety distances to the runway to minimize the risk of radio signals bouncing off their fuselages and sending a blind-landing aircraft astray. This can have bad effects for capacity, so in normal operation landings are done with a nonzero decision height.

In fact, most airports don't spend the additional maintenance cost needed to certify their ILS systems for zero decision height ("category III" ILS operation), or do it only for some of their runways. In case of bad weather at busy times, some flights will just have to divert to other airports.