In brief, LNAV/VNAV (managed modes for Airbuses?) are not always appropriate to use. Although they are normally very efficient, accurate and allow for a high degree of automation over extended periods of time, the downside is that they are somewhat inflexible, or require some time and effort to program correctly, in order to get the desired output.
The basic modes (such as, on Boeings, VS, LVL CHG, HDG, VORLOC, SPEED etc, I am led to believe these are referred to as open modes on airbus?) are normally appropriate to use to achieve short-term goals. Their operation is simpler and allows for a high degree of flexibility, at the cost of some increased in vigilance required.
As a pilot, training will teach you what each mode does. With experience you will decide when each mode, and thus what level of automation, is appropriate.