I'm not the person you asked, but here's my reasons.

I'm on Linux and there no longer is a supported version of Flash by Adobe. (Apparently there is some NPAPI to Pepper bridge though, to get Chrome's Flash working in other browsers.)

I already uninstalled before that, because I wanted to force HTML5 for sites that dynamically switch between Flash and HTML5 and because I wanted to nudge more sites to support HTML5 by boosting the stats of people who don't have Flash installed.

I think everyone got most of my reasons covered below, but here they are: no need to manage updates, smaller security surface, battery life, fan noise. I typically browse with Firefox Nightly, and without flash installed globally I get HTML5 or whatever a site wants to offer me if I don't advertise flash compatibility. If I hit some must-have flash I have a key combo that opens the current tab in Chrome.

Historically, it's tended to have had far more zero day exploits than the web browsers themselves. removing Flash removes a huge attack surface, and 95% of the time you don't need it.

I don't have flash installed (usually using Safari and Firefox) and have to jump over to Chrome to use a site maybe once or twice a week, which is more than acceptable given the insecurity of Flash.

I really don't like flash. It's full of security holes and its primary uses annoy the living hell out of me. So I only use a browser that has it when it's on my terms.

Then you enter the toxic hellstew of Flash Player updates.

don't need to install. Now its default in browser