Each of the two levels of government must therefore have specified areas over which they have power and, as a consequence, specified areas over which they don't. In addition there's a category of shared responsibilities. The Constitution attempts to decide who does what before anyone has to ask. Generally, the federal power makes laws that affect the entire population and land, the smaller units makes laws that govern their own area and citizenry. Unfortunately, it sounds a whole lot easier than it is.
Some topics overlap and some require a seemingly arbitrary judgement call.
In the States they decided that Criminal Law was for each individual state to control while in Canada we have a federal Criminal Code. Then, because constitutions aren't written by prophets, there must be a means of amending them. Circumstances change and decisions made in good faith may not keep up. As time passes and social needs takes forms previously unimagined, many of the reasons for the original division of powers may no longer apply. As well, new and unanticipated situations arise.