SharePoint crawl is the tool that creates the index to be searched against. An index is simply "a set of items each of which specifies one of the records of a file and contains information about its address" (Google definition).
The content is all there as soon as you click "Save", but the crawl is the thing that looks at new (or modified) content, parses the metadata and keywords, and creates connections with other existing data.
The reason to do interval crawls instead of a continuous crawl is for efficiency and server-load reasons. As you mentioned in our conversation, the following scenario would not be an efficient use of resources:
A user makes a change to a page, then again and then again. A crawl at, say, every 30 minutes would find maybe the final form (and index the page once) while with my suggestion the page would have indexed after each iteration (so n times).
To add to that, if it were to occur in the middle of the day in a large company, the servers may already be under load from other user interaction, and to crawl each and every modification (possibly tens of thousands per minute in a large enough company) would place significant additional load on the servers.
HowStuffWorks has a great breakdown of how/why search engines work, including the purposes and functions of crawlers. The same principles would apply to SharePoint as well.