Lowe taxes, insurance (health, auto, life, etc.), and other financial services.
In addition, all the people that you are alone. Do you agree or not? Should not all singles have the same financial benefits as married couples?
Since ancient times it was to promote the interests of government, marriage, as it creates a more stable environment to govern. Married people settle down, buy nest couple, married a flat, married people are more consistent tax sources. Easy to move people, single people are transitory, tend unmarried property leasing, people have only a family income below. Accordingly, some government offers incentives for married people, do not offer to individuals.
If individual people who have access to these incentives, they can get married.