A lazy web developer is a good web developer – that is, a clever programmer will save themselves lots of mundane work by automating simple processes. An obvious idea is to write an excellent META Description and use it automatically on every page of your site – better than nothing right?
The problem is, it’s possible the search result for every page on your site will come out looking the same. This is not what you want – you want your site to cover a range of topics and search phrases, and having 1000 pages that look the same is just asking for Google to treat them as duplicate content.
If you aren’t able to create a unique META description for every page, it’s best to leave it blank and let Google create a snippet out of page content. In all fairness, the search engines do a pretty good job of creating relevant snippets. So you should only replace a snippet if you are going to be improving it.