Free will is indeed a complicated subject, but this idea of what it would give you if it was real by comparison to what you wouldn't have if you didn't is at the heart of the real philosophical debate, in my humble opinion.
Most modern physicists who believe that the universe operates by deterministic principles will tell you that free will is an illusion. This is an important point because the mathematics of a deterministic universe and our own empirical experiences and observations would seem to be at odds on this matter.
Put simply (only to give context to both OP and this answer) physical laws rely on a concept called symmetry in order for us to apply them as commonly as we do. Symmetry in this context has many special meanings, but the most important one for us is that with the exception of the Second Law of Thermodynamics (Entropy), all physical laws work BOTH ways through time. If you take billiard balls and simulate a collision, then take the end points of those balls and apply kinetic energy to them in exactly the reverse situation, you'll get the starting position of the balls from the first collision at the end of the second.
The point of that is that if you extrapolate, that implies that if you know enough information about the universe at a given point in time, AND you know completely the 'laws' of the universe, then you can accurately extrapolate what the universe will look like at any other point in time, including the future. If you can do that, then we can't change that future by making a choice that is different from that which 'always' was supposed to happen. Put more simply still, fate is a thing. That thing is also called static space-time, but that's a more detailed discussion that has been held on this site previously.
The thing is, if that's the case, then while empirically it feels like I've chosen the chocolate ice-cream and not the vanilla, that choice wasn't the result of free will; it was the choice that was foretold by the universe's arrangement of neurons and energy within my brain set at the beginning of time that made the choice and it only feels like I could have chosen the vanilla, a choice that I was never going to make. This is why physicists call free will an illusion; it feels like it's real but isn't.
If free will is real however, it would give us the most precious gift imaginable; the ability to literally change the universe by choice. This ability comes at great cost; most of the deterministic laws we believe govern the universe and that are responsible for our development of things like electricity, space flight, and even this forum on the internet are (at best) only MOSTLY consistent, and/or reliable. It would mean that while our physical observations indicate that the universe operates deterministicly in the main, our ability to make free choices that go against that deterministic 'programming' of the universe implies that at least some of the universe has to be non-deterministic in nature. After all, a deterministic universe cannot contain non-deterministic functions, but a non-deterministic universe CAN contain mostly deterministic functions as determinism is a subset of all possible actions.
As such, assuming that the laws of physics are still deterministic in nature, free will would give us the ability to override those laws and choose a new direction for the universe at every step. Some of those choices (like chocolate v. vanilla) don't have that much of an impact. Others (like burning coal for electricity generation on global warming scales) will have greater impacts. Either way, it's our choice and it alters the universe subtly each time we enact such a choice. That can only happen with a dynamic space-time model, with all the problems that creates for our modern understanding of physics.
So; no free will gives us a simpler model of the universe that complies with our existing mathematics, but means that those choices we make aren't really within our power to change and that goes against what we experience. Free will as a real thing on the other hand makes our understanding of the universe more complicated and opens several cans of worms, but bestows on us a terrible responsibility to make better choices whenever we can, which is in line with our experience.
Which is right? I don't know and I suspect I never will but to answer your question as specifically as I can, free will would give us complete responsibility for our choices. Given that I don't know if I have free will or not, I personally choose to act as if I do have that responsibility and as such I shouldn't be able to go wrong either way.