Design templates are a great way to get a polished design without needing to hire a designer.
At the early stage of building your product, you dont need a custom design. Your initial focus should be validation. Find out if you solve a problem, and find out if its a big enough problem that people are willing to pay for to sustain your business.
Once you have the foundation in place, good design does help you stand out compared to using MaterialUI, Bootstrap out of the box. At this point you can hire a designer, or work with a contract designer to get a block of work you need done.
You can also pair a design template with an existing framework that you modify to save time on both design and development. Or you can implement a design template and build your own baser components that you need and arrange them throughout your app.
A design template will save you time, money and bring a polished look to your app without distracting you from what is important - validating your idea, and finding customers.