Okay, i'll leave out MBGP (Multiprotocol BGP) which is primarily used in an MPLS VPN environment to exchange VPN routes between PE devices. This is a specific use but i'll cover the more obvious uses of BGP generally.
I will also be covering it primarily from a non-ISP perspective ie. ISPs use BGP to exchange routes and influence traffic coming in out/going out of their AS. They have to use BGP. But for a business it may well be different.
BGP as you say is used to influence traffic paths so it really depends on whether you have multiple paths for the traffic to take. Like i said, if you only have one exit point from your network there is little use for BGP because there is nothing really to influence ie. to get in or out of your network you go the same way.
Where BGP becomes useful is if you have multiple exit points from your network and you want certain traffic to go one way and certain traffic to go the other. Then you can use things such as local preference/weight/MED to influence not only how traffic leaves your network but also how traffic enters your network. BGP gives you the ability to advertise your routes with certan attributes attached and is it primarily this functionality that allows you to direct traffic via different paths.
BGP is however a complex protocol so it should only really be used when it is needed ie. if yu only have one path out of your network then you could run BGP but why bother when a simple default-route in your IGP would achieve the same thing with a lot less configuration.
Jon
