Replies

Bump - used when someone wants to post a reply to a thread
simply to "bump" it back to the top of the "Latest Posts" list...
...so more visitors will see it.
"Bumping" a thread also helps to "feed the Google-bots." ;)

From support.google.com:

Googlebot
Googlebot is Google's web crawling bot (sometimes also called a "spider").
Crawling is the process by which Googlebot discovers new and updated pages
to be added to the Google index.
We use a huge set of computers to fetch (or "crawl") billions of pages on the web. Googlebot uses an algorithmic process:
computer programs determine which sites to crawl, how often, and how many pages to fetch from each site.
Googlebot's crawl process begins with a list of webpage URLs, generated from previous crawl processes
and augmented with Sitemap data provided by webmasters.
As Googlebot visits each of these websites it detects links (SRC and HREF) on each page
and adds them to its list of pages to crawl...