As a beginner, it’s not just necessary; it’s absolutely critical. Outlining your structure, turn of phrases and break-up into paragraphs gives you an idea of the flow and logical sequencing of thoughts. It also helps you identify vacuums or logical double-steps you may have inadvertently taken during your outlining. It helps you edit your document for flow and sequencing.
Even for experienced writers, it helps, if the volume of writing is large. I would definitely recommend outlining for all writers. Only after you gain sufficient experience, read 10 years plus of daily writing, you may choose to do away with it, and that too - only if you’re comfortable.
The flip side - it sometimes interferes with your flow of thoughts, especially when you have mulled over the subject for a sufficient length of time to let it gush down onto the page.