In the home bilingualism is usually the result of families made up of two distinct linguistic groups, for example one parent may be Spanish-speaking one parent may be English-speaking. Some families -- either through deliberate choice or through organic processes -- end up having their children brought up mono-lingual. Other families decide consciously to be bilingual, or it simply happens as a natural result of two different languages being spoken. Usually it is a conscious decision, and this is often a desire to give the child or children equal exposure and this equal benefit of both cultural and linguistic heritages to which they belong.
Bilingual schools or classrooms are common around the world and in many parts of the United States. While bilingual children gain many benefits from bilingual learning, bilingual schools also often show up higher in SAT tests than monolingual schools. In the United States, the most common bilingual schools are those in which English is taught alongside Spanish, though there are also bilingual schools that teach minority languages from minority ethnic groups such as various American Indian tribes.
Bilingualism is a valuable skill to have in today's employment marketplace. To have fluency in two languages is very marketable and opens new horizons and opportunities to people than if they were monolingual. Obviously, if someone speaks English and Spanish she can seek jobs within two vast markets instead of just the one. Furthermore, there are jobs in each marketplace where knowledge of the other language is needed, i.e. translators or teachers. There are cultural benefits to being bilingual, as a person can converse socialize with many more people from different groups, rather than being confined to their own linguistic group. Students or children from marginalized or minority languages don't feel excluded, and feel proud of their own culture and remain connected to it.
Some argue that bilingualism -- especially in schools -- is costly and wasteful. It takes a lot of time and resources to operate bilingually, and learning another language fluently is no small undertaking. If learning this language is ultimately unnecessary to future prospects and success, then the resources devoted it teaching it could be considered wasteful. Furthermore, it may even lessen proficiency with one language -- or the primary language -- at the expense of a more moderate level of proficiency in both languages. Politically, bilingualism can also be controversial as language is closely associated with race and culture. Some see bilingualism as being unnecessarily socially divisive.