We can divide a bar magnet into a large number of tiny bar magnets. Evaluate, the magnetic field as the superposition of the contributions of all small aligned magnets present in the bar.
For a point near the centre of the bar, the magnetic field is very small because the contributions of the nearest magnetic dipoles (tiny magnetic bars) is partially cancelled by the rest of the dipoles because its contribution is anti-parallel.
Actually, for a large bar magnet the value of magnetic field is negligible at the centre. On contrary, near the edges the magnetic field is greater because there is a minor quantity of dipole magnets to oppose the contribution of the nearest.