Short answer: Publication support can be worthwhile when language, formatting, journal-fit research, or submission administration is taking time away from the research or creates a clear risk of avoidable errors. It is less useful when the manuscript is already publication-ready and you only need information that the journal provides free.
Buy a service for a defined problem: language editing, journal selection, formatting, reviewer-response clarity, or submission preparation. Do not pay merely because a provider implies it has special control over a journal or indexing database.
Compare the cost with the time and expertise required to do the task yourself. Researchers with strong academic English and journal-submission experience may need less support than first-time authors.
A good provider should help you understand the scope before purchase and should never turn a support service into a guarantee of publication.