1. To talk about a time period that is not finished (today, week):
- I've written a rough plan this morning (it is still morning).
2. To show that something happened at some point in the past before now. We don't state when it happened:
- I've collected plenty of information (at some point before now and I will use it to write my essay).
If we state when something happened we must use the simple past:
- I wasted a lot of time last week.
3. To talk about a present situation that started in the past, usually with for/since:
I've worked really hard for the last two weeks (I've worked hard till now).
4. To talk about something that happened at an unstated time in the past but is connected to the present:
- I've read all the books on the reading list (I have the notes now).
(c) Cambridge Grammar for IELTS