When do I use present perfect tense instead of the simple past? [duplicate]
It's a subtle distinction. The present perfect describes the present ... right now, you can be described as having finished your work. The past describes an action that took place in the past ... five minutes ago, you finished your work.
I've finished makes a more important connection between the action of finishing and the situation the speaker is in.
For example: I've finished my work so we can now go to the park.