Indexation of a journal is considered a reflection of its quality. Indexed journals are considered to be of higher scientific quality as compared to non-indexed journals. Scopus is a bibliographic database containing abstracts and citations for academic journal articles.
One probable reason is that institutions do not want to associate themselves with any publisher that might be fraudulent - as in those that do not even have a true editor, but only a desk with a computer on it, and where the only thing the "editor" does is to count the money, and tick the "Accept" button once the check has arrived.
So the indexation is a way - while not a terribly accurate one - to distinguish lousy science from - at least occasionally - rather good science.
The indexing publishers are establishing criteria for inclusion in their databases, and the quality of the indexed journal is known and also its visibility, and that is the reason for institutional choice of journals. In times of information overload, this selection process in important.
Well, not only at Scopus. In the case of Colombia in a wider group of databases. But, the most important for the national agency of Science (Colciencias), are Web of Sciences and Scopus, and among them, according the quartile position, Q1, Q2, Q3 and Q4.