There are many different reasons. In my case I use it mainly to save money. Mobile operators have usually very cheap calls inside their own net but it is very expensive if you call to other operator. But there are also many other reasons why people want dual-sim.
1) You can have a company phone number and your private in one phone.
2) If you travel a lot between 2 countries, you can have different sim cards for each of the country (saves you a lot of money).
3) You can have internet service on one of the card and use the other one for calls.
4) You can have a secret number that only a few people know and you can temporary disconnect the other card if you don't want to be disturbed.
5) No operator has 100% land coverage, with 2 sim cards you can reach at least one of the networks almost everywhere.
There are many other reasons why it is better to have 2 sim cards. But I believe these are the most common.