I just introduced the concept of adding and subtracting negative numbers to my students and I found that I cannot come up with a good explanation of the use of bracket. In particular, I ask my students to 'add $-3$ to $2$', some of them give me: $$ 2 + - 3$$ I told them to add a bracket like this: $$2 + (-3)$$ but they start asking why. In fact, when I recall my maths lesson in high-school, we seldom, or even never, allow the four signs of operation ($+$, $-$, $\times$ , $\div$) to be 'next to each other'. For example, $8 \times -3$ should be written as $8 \times (-3)$. But I was never told the reason behind this.
Of course, if we are dealing with $2+3 \times 4$ and $(2+3) \times 4$, the necessity of including the bracket is obvious. But there seems to be no ambiguity in the cases I mentioned above. So why do we have to include the bracket? Is it a rule? Or is it just a practice we follow? Thanks in advance.