Because investors can enter and leave at NAV, an open-ended fund must value its portfolio regularly and hold enough liquidity to meet redemptions. Its offering documents set the dealing frequency, notice period and any gates or lock-ups that manage that liquidity.
The alternative, a closed-ended fund, raises a fixed pool of capital for a fixed term and returns it as investments are realised. Investors cannot redeem; they can only sell their interest to someone else, if a buyer exists.