The importance of using dateModified in your structured data or schema markup is that it helps Google update your content date on search results. This has an effect on the click-through rate as some users look at the date an article was published in search results before they click it.
There are articles that need to be new like deals and discount dates as visitors would want to click on the latest deals and discounts.
Here is an article that explains why publication date matters for SEO.