Tribal pacts can help you in developement of your civilisation and sometimes even during wars with other tribes.
To become friends with other tribe you have to create some kind of diplomatic corps. Select your chief and 3 to 6 normal creatures. Give them insturments and go to chosen tribe.
When you talk to selected individual from chosen tribe, he would show you what kind of instrument he would like to hear. Give them what they want to get their favour.
Do it required amount of times to succed.
But what if you want to make friends with a tribe that doesn't like you? Well, the best way is to give them food. To hand them 15 units, press right mouse button on the selected village's pile.
Making friends with other tribes gives you: developement of your civilisation and your allies would send you food.