WordPress users use "admin" username by default to login to admin panel because WordPress suggest this username at the end off install. But "admin" username is the most common username to brute-force because this username exists on the most part of sites powered by WordPress.
WordPress does not limit login attempts by default. You can limit login attempts with "Limit Login Attempts" plugin. I installed this plugin on my site and I get about 1 brute-force attack each 3-5 days and every time the brute-forcers try to use "admin" username.
Is there any benefits of using "admin" username by default?