Différence between "charge" and "durée"

"Charge" (work) is quantitiy of Work.
"Durée" (duration) is delay, in open days.

If 2 resources share 20 days of work (charge) to do on a task, with equal repartition (10 days each), duration (durée) will be 10 days, and delay will be 2 weeks (14 days or 12 days if task starts on monday).

If I work with ASAP planification method, I just fill "durée" field and it is enough.

NO.
Duration is informative unless you choose "fixed duration" planning mode for the task.

So, in which case can I use "charge"?

ALWAYS
You should always define work as ProjeQtOr is work driven : planning is calculated from work data, and not from duration. It is the only way to have realistic planning.
Imagine you have a resource working 3 days on Task A and 2 days on tasks B, and tasks don't have a dependency (resource can work indenpendently on any task). If you work with duration, global duration of the project is 3 days (the longest duration). But if you work on work, total duration of project is 5 days.
This is a simple example, but imagine working with 50 resources, working on 100 tasks, some with dependencies, some not, and some resources sharing some tasks, .... Working with duration will not help. Working with work will compute realistic planning.

Moreover if you have 2 tasks, each with duration of 5 days, task A is complete, task B half completed (50%). What is global progress of project ?
You should say 75% (50% for task A, 25% for task .
But now if I say that task A is a 0,1 day per day task (some management for instance) and task B is full time for 2 resources, real progress is 52% = (0.5+5)/10.5.
And if resource estimate that they need 4 extra work days to end task B (reestimate left work), then progress is just 38% = (0.5+5)/14.5

It is work driven (also called effort driven) planning.

Just because you did not run planning calculation : planning is not automatically calculated as it is considered as a management act.
Use button on top left of planning view (with the 2 gears)