1P+N MCB

In a single pole + neutral mcb... the single pole is used as over-current protection and the neutral leg is used as a disconnect to isolate the neutral, this would be used if you have 2 different sources of power feeding the panel(such as an emergency generator and utility grid or other) and didn't want to mix the neutrals as a safety measure....Generally this is required by code...

A swimming pool one example,but there are many.

A pool disconnect must disconnect ALL conductors.

Hazardous locations also,which include service station pumps.

The list is long.Many exceptions and local variations.

Always consult the local authority having jurisdiction(AHJ).

when is it a must to install a 1P+N instead of a 2P?

When the code requires it.....lacking that, then when common sense dictates....

Perhaps it also is influenced by what type of circuit breaker you are using. A 2P breaker that mounts on the panelboard bus is electrically very different from a 1P+N breaker that is lug-lug or a 1P+N breaker designed with the N being lug-lug while the P is attached to the panelboard bus.

--JMM

...and codes vary between countries.