There are at least two ways to make this argument.
First, we can argue it from a practical point of view. We can say that society is better off when women have equal rights and opportunities. A society that does not allow women equal opportunities is essentially abandoning the potential of half of its workforce. Women, if given opportunities, could contribute a great deal to the country's economy.
Second, we can argue from a moral/ethical point of view. Women are not inferior to men. Since they are not inferior to men, there is no reason that they should not have every opportunity to do anything they are capable of doing. It is simply immoral to deprive a group of people of their rights and opportunities simply because tradition holds that they are in some way inferior.