system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some sites check my browser before giving me access to it ?
gpt
Cloudflare is usually the one doing this. The purpose is to lock out "users" that the site doesn't want. Usually bots, web crawlers, and things of that nature. It also protects against things like DDOS attacks and other web based denial of service stuff. 

Or, super TLDR version, it's making sure you're an actual user with an actual browser, not a script sending traffic.