They’ve been moving people over without notification in the last few months and it has caused quite a few problems for people. Most recently there was an issue with splitting the devices and the hub into two different locations and then nothing would work. It’s been discussed in the forums, was recognized as an engineering issue, and they’ve since fixed that one. But the other problems can still occur, just depending on the account’s configuration.
See the community FAQ for the list of some of the things that can happen,
As for why someone would use the new app, it’s now the only way to get integration with the Samsung smart televisions and appliances. You also need it if you are using the Wi-Fi mesh hub or the V3 hub. It has a different SHM with a built-in delay. And, the rules engine in the new app is noticeably better.
The following two threads might be of interest 
:
SPECIFIC differences between "SmartThings Classic" (the V2 app) and “SmartThings (Samsung Connect)” (the V3 app)
.
Things I like BETTER about the new app vs Classic