The simple answer is "to store and retrieve data."

However, I assume the real meaning of this question is ".. why people use FileMaker rather that some other program," which in turn (given the realities of market share) usually translates to "why people use FileMaker rather than Microsoft Office." If this interpretation is wrong, please correct me.

Given that objective reviewers consistently find that FileMaker is better than Access for just about everyone and just about every application, the question ought to be "Why does anyone use Access?" The answer is simple.

About 85 percent of the computing world, higher in business, uses Microsoft Office under Windows. Most of these folks got it for Word, Excel, PowerPoint and/or Outlook, but they got Access in the bargain. At some point they need a simple database. They have Access, so they try it. Access is not a bad program*. It gets the job done.

The next time they need a slightly more complicated database, they know a bit of Access so they learn a bit more. This continues until they're pretty good at Access and doing fairly complicated stuff with it. Again, Access is not a bad program, so it does that stuff without a major problem. They're happy, and they never consider another database management program.

The problem is that, had they bit the bullet and bought FileMaker in the first place, they would almost certainly have saved a great deal of time over the years and probably ended up with databases that were easier to use, easier to modify or enhance, and ran faster (which can be important when files get large). However, they were like the proverbial frog in a pot of cold water who doesn't notice it getting hotter, bit by bit, until it's cooked. They never had any reason to look around because the incremental steps they took in Access never reached the required pain level. And now they are, if not cooked, at least hooked.

Another reason people use FileMaker is that it's multi-platform: databases can move easily between Mac and Windows. While Macintosh market share is small, a company doesn't need to have many for data sharing to be valuable. Access doesn't run on Macs and probably never will.

This isn't a religious debate, though people try to turn it into one. It's a business issue. I don't deny that there are databases for which Access is the right choice on its merits, though I think they comprise a minority of Access uses. (For one thing, if you want to sell database applications in some markets, it makes sense to sell databases that run under a program that most of your target market already has.) Those are the facts as I see them. I'd be happy to hear other opinions.

(Back to the original topic of this thread: I got FM Pro 13 yesterday, sent download information and license key to wcj1, and he sent me half the cost via PayPal within minutes. So far everything works as advertised.)
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*I know Access fairly well. Wiley published my set of exercises for it as a supplement to a recent information systems textbook. In addition to FM Pro 13 running under Mac OS X 10.8, I also have Access 2013 running under Windows 7. I've developed non-trivial databases in both. I think I'm both knowledgeable and objective here.