Past tense vs present perfect tense

Question

I have queries about past tense and present perfect tense.

1. I have made a appointment to see the university careers advisors immediately after the seminar,So I will need to leave on time.

2. I have done lots of presentations before and I feel OK about this one.

I got this two line from a grammar book. Now my doubt is Why we use present perfect tense instead of past tense in this two statements? I mean, both describe past activity.

After consulting various documents I got

Present Perfect vs. Simple Past

We must always use the Present perfect when the time of an action is not important or not specified.

We must always use the Simple past when details about the time or place that an action occured are given or requested.

One more thing

Can we use I have to leave on time instead of I will need to leave on time.

Answer 1

The Present Perfect is a tense that connects the past with the present. We use it when the time of an action is not specified/important AND when there is an immediate connection with the present. In the first statement, the important fact is not when you made the appointment but the result of that action, that NOW you have an appointment.

In the second statement, you are talking about something that is a part of your life experience, which is the metaphorical bag we all carry with us in the present.