Why using selenium grid hub over appium functionalities for parallel testing

Im abit confused is it really major difference to achive parallel testing using selenium grid over just using same appium server using different port and bootstrap, wdalocal port, …etc

I have performed parallel test using the two techniques and always the way using selenium grid is always slower

Can you please provide more info whats the actual benifit pros and cons using selenium grid over appium functions (port number ,wdalocal port , …etc)