Having an optimized GMB can help in two major ways. firstly, it simply lets your customers know who you are, what services you offer, and most importantly, what others think of your business (reviews).
Secondly, optimizing your business page can greatly help search engines find your business and rank it over your competitors. This makes it much easier for potential customers to find your business, which in turn greatly increases the likelihood of them choosing your business!
simply put, they can't choose you if they can't find you!