[Asterisk-Users] RealTime Vs. AGI and PHP or MySQL calls within extensions.conf

[Me]

I may not understand fully how any of these three features work but...

Can someone tell me what benefit there is to using RealTime instead of say 
calling a MySQL database directly from the extensions.conf using the built 
in MySQL functionality? Also, it looks like I could use PHP via AGI to also 
lookup extensions?

Am I totally lost?

Thanks!