Change is the result of decisions.
Business analysts are change agents. Part of our role is to help facilitate good decisions.
A good decision is based on a strong understanding of what the different choices mean within the domain they are in. Facilitating good decisions is achieved by having a strong domain understanding that allows us to extrapolate what the future looks like based on the different choices that can be made.
Understanding the domain is more than just having the knowledge of a domain, it is having frameworks and models that help predict outcomes. Decisions are made to produce outcomes, so understanding what those potential outcomes are is key.
The role of a business analyst is not to make the decisions, but to gather and communicate the relevant knowledge and understanding so that a good decision can be made.