The wall would basickly end the problem of people illegally entering from the southern border. Hundreds of thousands of people enter the US this way every year so it would be wise for us to fund the wall to stop that. Trump knows that the only way he can do it now is with having the government shutdown over it.
If he can't get Mexico to pay for it maybe he shouldn't have made this bs promise in the first place. Either way even if the wall appeared over night it still wouldn't fix anything in the long run. The majority of illegal immigration is a result of people entering the country legally then over staying their visas and that's been the case since 2007.