Updating the content of your page titles and meta description can have a significant impact on your Shopify store's SEO. Lin asks a related question:
Does it matter that I write the keywords in the "Page Title" section or the "Meta Description" section? Can the words be repetitive for the 2 sections?
What you write can have a significant impact.
The Title becomes the actual page title people see in their browsers (the tab name) and the headline in the search results (blue link). The Title can also impact your rankings depending on what phrases and keywords you use.
The description is invisible on the page but it can be used as the description in the search results themselves, under the Title and url. Meta Descriptions don't impact rankings since 2009 but can have an impact on your traffic.
What happens is that the text from those two fields are what the searcher sees (your potential customer). So if you add some attractive and enticing text in there, it can convince them to click your link out of the other 9 in the search results.
Use keywords but make sure to focus on compelling copywriting that will convince your customer to click your search result out of the other 9 on the page.
Using the same keywords in both the title and description is good, as long as they make sense. If you had to choose between using a keyword or making sense, err on the side of making sense.
At the end of the day, it's the clicks that count and not the rankings.
This idea of making your search result stand out is the major benefit of Rich Results and Rich Snippets.
Instead of just seeing the usual blue link, green url, black description; Rich Results can add images, orange review stars, and product details to your existing results. This can lead to more clicks and thus, more traffic.
Most Shopify themes don't quality for Google's Rich Results due to bugs or missing structured data.
JSON-LD for SEO provides the high-quality structured data that will let your store qualify for Rich Results. Often times faster than the industry averages.