I've changed the meta description tag in many of my site pages. Should I select Fetch as Google->Fetch->Add to Index->URL and all linked pages in GWT?
Will Google recrawl the whole site faster than it'd normally?
This option can be used maximum of 10 times in a month. Can there be any side affects when doing this way in case only a few of site pages have changed?
It depends how much in a hurry you are... If you can wait, it will happen naturally next time google crawls you (you can estimate by doing google search for site:yoursite.com, and clicking on "cached" to see when it crawled you last time, and extrapolate. Or be more sure by looking at your access logs for googlebot)
If you are sure you haven't messed up (recheck via rich snippets tool), do submit refetch - it will get your content faster. But still be prepared to wait some days for results to start showing up (and they won't show up all at once...)
I have already answered this question at the address listed below. I'll provide the answer in here, too. This information is current as at May 10, 2018:
Login to Google Search Console.
Go to Crawl > Fetch as Google > Enter URL > Fetch and Render.
Wait a little while. A loading indicator will appear in the list below under the Status column. When the status changes, a button should appear in its place. It will either say "Partial" or "Complete" (there may be other statuses I haven't seen before). You should see a button to the right of that status that says "Request Indexing".
Click Request Indexing.
To address the second part of your question, When you click Request Indexing (after you've clicked Fetch and Render), Google will present you with 2 options:
Choose the second option if that suits you best. For example, you could enter your homepage URL, and if it directly links to all of the pages you want Google to re-index, then that second option is the one you should be using.
Sometimes it takes time for all of your pages to be indexed by Google. Google might even decide not to index every page on your site.
For example, I requested my entire site to be indexed. But only 496 out of 562 pages were indexed. However, several weeks later I began to notice (without any changes by me) that more pages were still being indexed. As to the "why" of your question, well, we need a lot more info from you to determine why not all of your pages were indexed. There are a lot of variables to consider.
Source: Best Ways to get Google to Re-index existing and new pages
