Use case is simple. If you are writing or researching something set in a historical period, it a calendar helps to see how events fit together. And sometimes you will have a date and need to know the day (and vice versa). For most European uses, the Julian and Gregorian are sufficient (though there are some specialist exceptions, and the timing of the switch varied between countries). It's less important the earlier you go (especially BC) because there were different dating conventions. Ditto for other areas and civilisations.
There are other ways of doing it (Aeon Timeline is almost infinitely adjustable, though it takes quite a bit of work), but it can be useful to have it all in one place.