In case you are wondering, buying into the fund (meaning adding to the fund's assets) doesn't increase the NAV because the company simply issues new shares to keep the NAV at parity. Similarly, when an investor redeems investment in a fund, the management company retires the number of shares necessary to keep NAV parity for the day.
Given that equities (and many other asset classes, such as bonds) are continually priced through the trading day, it doesn't make sense for the management company to price the NAV through the day continually, which explains why the funds are priced and traded once a day.