Google has been using snippets automatically generated from websites to show as instead of the Meta description. Only until recently, the last few years, have Google and the other search engines began to recognize the use of custom generated Meta tags that webmasters have inserted into the HTML code to show up in the search engine rankings. The search engines in their early stages would pick and chose parts of the website and use that as the description that showed up in the search engine results. If the tags are left blank the search engines will chose a snippet from the page and use this as the description.
It is always advisable and suggested by search engine optimization professionals to take advantage of and use Meta title and description tags on every page on your website. DO not use the same title and description on every page as this is not descriptive and will not help internet searchers.
The reason search engines changes their algorithm to allow the spiders to remember the meta description tags is for webmasters who want to increase their click through rates. Webmasters can tweak and change what shows up in the search results to maximize click through rates (CTR) from browsers and reduce bounce rates. Make sure to use a highly descriptive title and description that is both human friendly and search engine friendly. What this means is use keywords, preferable long tail keywords, but do not over use keywords to the point where it become unreadable to humans. Make sure everything flows smoothly and makes sense.