Search Engine Optimization, or SEO, is the main factor in how the use of metadata has changed. For instance, the practice of flooding the keyword tag with every slightly relevant search term has led Google to almost ignore this aspect of metadata on a website. It is still acceptable to include this tag in your overall SEO approach; however, it is nowhere near as important as it was several years ago.
The description metatag is still very useful as Google may use this to rank some pages, but it's also used when your website is displayed in search results. Because of this, care should still be used when crafting your descriptions.
There are other metadata settings to understand as well, such as robots and author, and these tags are possibly becoming more important as things change online.
Metatags and SEO are explained in detail in our premium Joomla tutorial course - Joomla Pro.