Lnav can just take care of your lateral navigation. Imagine a 3d map spread out before you and you’re a toy airplane. The Lnav can get you anywhere on that map but you have to manually control the climb and descent to land safely, otherwise the autopilot flies you straight into a mountain. Add Vnav to that and it gets you safely over all obstacles if you programmed the altitudes into the waypoints and also determines the best climb/descent profile based on the selected waypoint altitude, (respecting restrictions, noise abatement procedures etc) and glideslope degree which usually is 3 (correct if wrong). Basically you can say that Vnav adds a whole dimension. Lnav = 2D, Lnav + Vnav = 3d navigation. Often times the Vnav mode can also just act as a guidance, when the plane lacks the full automation.
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