The difference between
She is leaving because she finished her work.
She is leaving early because she has finished her work.
is, in the first sentence she might be leaving early or at the regular time whereas in the second sentence she is definitely leaving early.
She is leaving because she finished her work.
She is leaving because she has finished her work.
Without further context, these two sentences have essentially the same meaning, if anything, possibly she left right after "she finished her work" (at the usual time) but maybe left some time after "she has finished her work".
She is leaving early because she has finished her work.
She is leaving early because she finished her work.
Also have equivalent meaning.