Adobe’s (or Macromedia’s, if you back far enough) Flash was a game-changer back in the day. It allowed far richer content and interactivity than was possible using plain old code. If you wanted any kind of animation or embedded video and audio on your site, Flash was pretty much your only choice.
Many business sites at one point or another used Flash to some extent. Some were even built completely in Flash, where the entire site – menus and all – was basically one huge interactive animation. Glittery graphics and flashy transitions abounded. Having looped background music was very cool (actually, no, it wasn’t).
Now, like horse-drawn carriages or AOL, it has reached the end of the line. The internet has evolved dramatically in a direction almost completely opposite from what Flash does. It has now become a liability to have it on your site and almost all that it offers can be replicated using better, more modern tools. Furthermore, its drawbacks tend to outweigh whatever benefits it provides:
Site speed
Flash animations are almost always bulkier than their alternatives. People may have been willing to stare at a “Loading!” bar for 10 seconds in 2002 but nowadays visitors expect a website to appear almost instantly. We don’t even want to wait for a video to load, much less sit through a gratuitous intro animation with a company logo flying around with fireworks in the background (if you have something similar on your website, please take it offline and email me right now).
Search Engine Unfriendly
A Flash animation is a closed box with a window on it. Google can’t see the nuts and bolts inside, which means it can’t properly index the site. The reasons why that’s bad are pretty self-explanatory, especially if you’re hoping to draw in new customers with a good ranking.
This is especially bad if your whole site is built in Flash – Google will only be able to index the homepage, and it will be almost impossible to rank for any relevant keywords. You may as well have no website at all.
Unnecessary
A lot of things that people have used Flash for should be avoided (like the aforementioned intro animation) and the rest can be done in other ways. If you want a fancy-looking menu with nice drop-downs and colours you can do it with CSS, which is much (much!) more efficient than Flash and still allows your site to be search-engine friendly. If you want video you can use an HTML5 player or embed a YouTube video (which admittedly uses Flash, but at least it’s hosted off-site and has no adverse impact to your own webpage).
So Now What?
If your site was designed in the early ’00s it almost certainly uses Flash in one way or another. Setting aside the fact that if your site is that old it probably needs a revamp anyway, you should especially do so if it makes extensive use of flash. If it’s just a video here and there then you’re probably fine, but if your navigation is all-Flash (or your whole site!) you really aren’t doing yourself any favours and should strongly consider bringing your website up-to-date.