The main advantages of logfile analysis over page tagging are as follows:
The web server normally already produces logfiles, so the raw data is already available. No changes to the website are required.
The data is on the company's own servers, and is in a standard, rather than a proprietary, format. This makes it easy for a company to switch programs later, use several different programs, and analyze historical data with a new program.
Logfiles contain information on visits from search engine spiders, which generally do not execute JavaScript on a page and are therefore not recorded by page tagging. Although these should not be reported as part of the human activity, it is useful information for search engine optimization.
Logfiles require no additional DNS lookups or TCP slow starts. Thus there are no external server calls which can slow page load speeds, or result in uncounted page views.
The web server reliably records every transaction it makes, e.g. serving PDF documents and content generated by scripts, and does not rely on the visitors' browsers cooperating.
Reference By: en.wikipedia.org/wiki/Web_analytics#On-site_web_analytics_technologies
