It decides on the execution plan to be used when a query is run. If this execution plan is sub optimal your query will continue to perform sub optimally. The execution plan is a best guess based on the schema and index statistics as the optimizer doesnt have time to evaluate all potential plans. The quality of the execution plan is only as good as the accuracy of the optimizer’s cost and cardinality estimations. If you don't have enough memory to keep all execution plans cached the optimizer will be run extremely often.
Even though it is extremely sophisticated the query plans it outputs are only as good as the information that is given to it. That is where miniDBA helps out by ensuring schema, T-SQL and environmental design and meta data are all as good as possible. The tools it provides help with this task and enable you to see performance benefits.