Because it’s there and it’s free, and web-page counters are so 1997.
First there were log files and only people who bought really expensive software could figure out what the heck the half-million lines of incomprehensible gobbledygook really meant. The rest of us used web-page counters. Anyone could see how many people had come to a page. As long as the counter didn’t crash, or corrupt its storage, or overflow and start again at zero, there would be a nifty little graphic of numbers that looked like roller skates (or pool balls or stadium scoreboard numbers or whatnot).
Around 1998, the arbiters of taste on the Internet (i.e., everybody) decided that page counters were so 1997 and that there must be a better way.
And also about that time, web site statistics packages or “stats” came into common use — not common use by huge businesses that could afford thousands of dollars for software but common use by us peons who rent our web space from hosting companies for as little as $5 a month. Stats packages basically collect data but leave you to analyze that data. So they tell you what happens; they just don’t put what happens into any type of business context.
If you have Windows-based hosting, you may have a Windows-specific stats package, or your host may use the Windows version of one of the open source stats packages. If you have hosting on a Linux web server ...