1. Limited internet speed - G729 allows you to maximize the amount of users that can simultaneously make calls. If you are using something like ulaw/711 you will be lowering the mount of simultaneous users and creating quality issues.
2. Shared Internet. You are using your internet for both voice and data. G729 allows you to lower the amount of bandwidth required for voice. You voice bits will get out/in quicker because less data to deal with. Your router also needs to be configured for QoS.
3. Your PBX is hosted. Even calls between extension to extension have to make a round trip. Again keep the amount of data that goes out/in small.
4. Connecting to gateways. Do the math - if you have a 10/100 connection from PBX to a gateway and your data between PBX and gateway is saturating the link you are going to have quality issues. Using G729 reduces the amount of data being sent between the link.
5. Using a good quality phone is also important with G729. It can reduce noise/background sounds to further reduce the amount of data being sent. My own personal tests indicate that using a Polycom 33o with G729 end to end creates a better call than using a Grandstream GXP2000 using ulaw/711 end to end on ideal broadband conditions.
PS. Don't buy Grandstream products - just used as example here. Grandstream products have quality issues/high failure rates.