Commercial airliners often use ILS as back-up even after accepting a visual approach. They may couple the ILS to the autopilot, or they may hand-fly the approach, at pilot discretion. Legally they are not required to use ILS when cleared for a visual approach, so it's just a way of double-checking things and not all crews/airlines will use it.
While an ILS is very accurate, it provides accuracy that isn't really necessary in good visual conditions. Airline pilots are quite capable of flying the aircraft visually to a very precise landing without ILS, when weather permits. Pilots are also capable of holding the aircraft on a localizer and glide slope by hand if necessary/desirable (even for ILS approaches in poor weather). Some categories of ILS approach require that the autopilot be used for the approach (e.g., Cat III autoland).
Sometimes specific procedures for a given airline may dictate how ILS is to be used, e.g., ILS always required as back-up even for a visual approach, no visual approaches to be accepted even under good visual conditions, etc.
Air traffic controllers like visual approaches because it lightens their workload when weather is favorable.
You cannot legally fly an ILS approach under VFR if the approach cannot otherwise be conducted visually. You can use the ILS as back-up if you want, or you can couple the ILS and fly the approach with the autopilot, as long as you are still under visual conditions and have the option of flying the approach visually at any moment. Legally it will be VFR and visual no matter how you actually conduct the approach in the cockpit, and in any case you must be in VMC to carry out the approach. In other words, as long as you are not using the ILS to carry out an approach that you could not otherwise carry out visually, it's okay (and unofficial, since legally it will still be a visual approach).
When you are VFR, you may be asked or expected to fly the pattern, which pretty much renders ILS moot. But if you are making straight in from far away, you might be able to use the ILS as back-up.
Also remember that when making an approach VFR or a visual approach under IFR, you are supposed to land by looking at the runway, not by following the needles. This could be significant, for example, in the unlikely event that the ILS is malfunctioning—if you accept a visual approach and then follow the ILS, and the ILS is not in working order, you could be in trouble if you don't actually watch the runway and land visually. (An unlikely scenario, but possible.)