When it comes to expressing an essential requirement, "is necessary" is a common phrase used in English language. There are several other synonyms that can be used to convey the same meaning. For instance, the phrase "is essential" stresses the importance of a need. "Is vital" also emphasizes the necessity of a particular action or behavior. "Is indispensable" denotes a requirement without which something will not function properly. "Is mandatory" implies a legal or obligatory nature of a task or responsibility. Lastly, "is imperative" highlights the urgency and criticality of something that is required to be done. Using these synonyms for "is necessary" can make language more versatile and express essential meanings with greater clarity.