Bloomberg has a massive network effect which makes it difficult to displace. Particularly when you move away from exchange traded products like equities and into areas like corporate fixed income, interactions are still conducted via bilateral discussion (i.e., online chat and text price dissemination). To be in the game you need to be where everyone else is talking. There is an ecosystem of price scraping and trade processing in Bloomberg which facilitates the whole trade and portfolio management process. I can route orders via Bloomberg to any broker and monitor execution of my orders in real-time. No the costs are not cheap but it is a lot cheaper than hiring several members of an operations team which would be necessary if this was all done manually.

Bloomberg is also like an operating system. For example, there are electronic execution venues in many types of instruments which use the Bloomberg as their front-end. This is very valuable. When you are a trader, screen real estate is critical. You can have 6 30" monitors and it still isn't enough if your tools are fragmented across 50 platforms. The more you can keep things integrated into a few core tools the better.

You are also paying a ton for ultra-responsive service. When millions or billions are on the line you don't have time to mess around on a help-line. On a Bloomberg you have 24-7 ultra-responsive skilled help who are responsive in around 30 seconds.

There are other reasons but end of the day if you are a pro then 30k/year isn't cheap but its a lot cheaper than trying to hack around with amateur tools.