Why use Have appeared vs appeared?

appeared is the past tense of appear. Appeared is used in the past simple tense which you use when you want to talk about something that happened in the past and is now finished. eg

The dolphins appeared just before sunset.

have/has appeared is called present perfect tense. This tense is used when you are talking about something that happened in the past and it continues now in the present (or it is important now).

The dolphins have appeared just before sunset everyday this week.