It is a simple question which might have discussed it her brief, but I might have missed to understand it.
These arguments about sensor sizes are
never simple. There are some people who become
very belligerent about their position. Expect the same here.
A full frame sensor is better in low light, it will give a shallower depth of field
This is not precisely true. Other things being equal ( and equal is hard to define in a way everyone will agree on ) they
can be better, but just being full frame does not automatically make them better in low light.
Likewise shallow depth of field.
There's also the issue of size. Full frame lenses of similar quality to the crop frame lenses have to be bigger ( in general ). That's part of the compromise you make. There's no free lunch here.
and it will capture bigger part of frame while taking picture (It will cover more area).
No.
If you're on a full frame system you use a lens designed to cover the image circle required for that system. If you're on crop frame you use a lens designed for
that system. A kit lens for a full frame system might be 28-70 and for a crop frame 18-55, but they both cover roughly the same field of views on the appropriate system.
These are three major advantage of full frame sensor in comparison to APS-C or four thirds sensor.
Well they're only advantages if they're useful to you and you're willing to accept the downsizes of s full frame system ( cost, size of lenses ).
You may think shallower depth of field is an upside, but in practical shooting the more shallow you get you loose two things : precise focus is harder to get ( shallow depth of field can get razor thin on crop frame systems, never mind full frame ) and in compositional terms people overdo the out of focus blur thing. It's a hard lesson to learn and many don't, but if it was as trivial as that to make a good picture cameras wouldn't have aperture control at all.
Your third issue is simply a misconception on your part.
What may be other major benefit to use expensive full frame camera ?
The maker's get more money for both the cameras and the lenses. That's a "major advantage" for them. Not for users, but for them.
For some people there's an ego boost in owning full frame. I suspect this is why some of them become so aggressive in their promotion of full frame.
If it is broad daylight, will full frame produce significant better picture/IQ also ?
Not even under a microscope.
A lesson I try and teach beginners is that about 90%+ of their "image quality" is down to the composition, tonality and "emotional content" of the image. This is what registers with the vast majority of viewers. Hardly anyone registers details and if you get those things right they'll almost universally not care about the details.