This is the basic of knowledge level, for anyone who wants to have an effect on a Six Sigma project (even if just allowing others to work around them, knowing the benefits). Yellow belts have a basic grounding in Six Sigma and can have small roles in projects, but nowhere near the impact that you can have using Green or Black Belts. They are generally the people gather data, leaving the in depth analysis to the higher belts.
Before White Belts existed, Yellow Belt was the base level, for people who wanted to know about Six Sigma, but not get too involved. Many companies still have Yellow or even Green as their base level, with White Belt often ignored (treated as more for solo learners).
Why get a yellow belt?
Yellow belts are great if you want people in your organisation to understand and be cooperative with Six Sigma, but not necessarily get involved. Six Sigma requires a buy-in from people throughout the organisation, not just the people directly working on the projects, so this is a useful course to get people on board, without taking up too much of their time.
This probably isn’t worth it if you’re a solo learner (I’d use white belt then go straight to Green, which looks much better on your CV), but is great to spread awareness around your company (for which white belt is probably too basic). If you’re doing it as part of a company, yellow belt is often your ticket to start getting involved in projects, so is worth getting it cleared.
Yellow belt will also give you some basic process improvement techniques, which you can use in the rest of your job, even if not as part of a Lean Six Sigma project.