It is claimed, by neuroscientists among others, that speaking two or more languages increases cognitive abilities and in some way re-wires the brain, as it were, in a way that positively affects how the brain works. And it's true that we tend to think of people who can speak two or more languages as being bright. Learning a language is, anyway, good mental exercise, not to mention its benefits in introducing you to other peoples and cultures. This is why all school curricula should include at least one compulsory language. Anyone who knows two languages will find that, in certain circumstances, they compete for position, the vocabulary of one getting in the way of the other. Often bi- or multilingual people find one language more suitable for expressing certain kinds of thoughts or feelings.
There is another view on being bilingual, not from a neuroscientist, but from someone whose business involves words and language. Brought up in a bilingual home, speaking Greek and French, and also fluent in English, she is old enough to remember when her native language, Greek, was also in effect two languages.