What is gotra ?

Gotra is group of people who were living around group of cows. (cow means go). In encient time cow, horse etc were considers as assets. Even people used to fight to capture them. So different groups of gotra were having different names. To expand their group they came up with a rule, we will not marry in same group, wedding will be allowed only in different groups.

reason was very simple, they wanted to make good relationship with other group.
So question is whether this system still relevant now?