Mostly just convenience. You could get pretty much everything offered on BB from other sources and maybe save some money as the BB terminal monthly price is pretty steep, but you would have to have multiple vendor relationships and spend a lot of time digging through all the various other services. BB aggregates a lot of this for you.
For example, data mining and backtesting. You could buy data from other sources or find some free online, but bloomberg lets you easily download it into excel in an easy to use format to start prepping strategies based on your research.
Another example is getting headlines and economic data releases faster than digging through reuters, wsj, etc.
Also, BB has a pretty rich job posting section for finance professionals when you sign up for their service. Although, now you could probably find a lot of those jobs on linkedIn. Before linkedIn and probably somewhat even now, BB was a way for firms to look for serious applicants. It lets them separate the average wannabe who is looking for a finance job on monster from those who show some commitment and demonstrate some success simply by the fact that they can afford to spend about 2k a month for the service.
I probably sound like a BB shill. Truth is, it was probably a lot more worthwhile 10 years ago before the internet gave people an easy way to access so much information for free and so quickly.